"Error installing rails" because "extconf.rb failed" on Ubuntu 18.04 - ruby-on-rails

Question

I'm failing to install Rails.
Background: My (brand new) system = Ubuntu 18.04, Ruby 2.6.0, latest rvm and bundler installed and updated. The first thing I did after installing Ruby was to do sudo apt install ruby-railties and sudo apt install rails...I don't know why I didn't use gem install. After that, I never have successfully executed rails new but I did once partially execute it, but it failed and that's when I started down the rabbit hole.
When I type rails -v I get this:
Traceback (most recent call last):
4: from /home/globewalldesk/.rvm/gems/ruby-2.6.0/bin/ruby_executable_hooks:24:in `<main>'
3: from /home/globewalldesk/.rvm/gems/ruby-2.6.0/bin/ruby_executable_hooks:24:in `eval'
2: from /home/globewalldesk/.rvm/gems/ruby-2.6.0/bin/rails:23:in `<main>'
1: from /usr/share/rvm/rubies/ruby-2.6.0/lib/ruby/2.6.0/bundler/rubygems_integration.rb:482:in `block in replace_bin_path'
/usr/share/rvm/rubies/ruby-2.6.0/lib/ruby/2.6.0/bundler/rubygems_integration.rb:462:in `block in replace_bin_path': can't find executable rails for gem railties. railties is not currently included in the bundle, perhaps you meant to add it to your Gemfile? (Gem::Exception)
When I try to execute gem install rails, I get this:
Successfully installed concurrent-ruby-1.1.4
HEADS UP! i18n 1.1 changed fallbacks to exclude default locale.
But that may break your application.
Please check your Rails app for 'config.i18n.fallbacks = true'.
If you're using I18n (>= 1.1.0) and Rails (< 5.2.2), this should be
'config.i18n.fallbacks = [I18n.default_locale]'.
If not, fallbacks will be broken in your app by I18n 1.1.x.
For more info see:
https://github.com/svenfuchs/i18n/releases/tag/v1.1.0
Successfully installed i18n-1.5.2
Successfully installed thread_safe-0.3.6
Successfully installed tzinfo-1.2.5
Successfully installed activesupport-5.2.2
Successfully installed rack-2.0.6
Successfully installed rack-test-1.1.0
Successfully installed mini_portile2-2.4.0
Building native extensions. This could take a while...
ERROR: Error installing rails:
ERROR: Failed to build gem native extension.
current directory: /home/globewalldesk/.rvm/gems/ruby-2.6.0/gems/nokogiri-1.10.1/ext/nokogiri
/usr/share/rvm/rubies/ruby-2.6.0/bin/ruby -I /usr/share/rvm/rubies/ruby-2.6.0/lib/ruby/2.6.0 -r ./siteconf20190116-22693-tbjpnz.rb extconf.rb
checking if the C compiler accepts ... yes
Building nokogiri using packaged libraries.
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers. Check the mkmf.log file for more details. You may
need configuration options.
Provided configuration options:
--with-opt-dir
--without-opt-dir
--with-opt-include
--without-opt-include=${opt-dir}/include
--with-opt-lib
--without-opt-lib=${opt-dir}/lib
--with-make-prog
--without-make-prog
--srcdir=.
--curdir
--ruby=/usr/share/rvm/rubies/ruby-2.6.0/bin/$(RUBY_BASE_NAME)
--help
--clean
--use-system-libraries
/usr/share/rvm/rubies/ruby-2.6.0/lib/ruby/2.6.0/bundler/rubygems_integration.rb:408:in `block (2 levels) in replace_gem': mini_portile2 is not part of the bundle. Add it to your Gemfile. (Gem::LoadError)
from extconf.rb:469:in `<main>'
To see why this extension failed to compile, please check the mkmf.log which can be found here:
/home/globewalldesk/.rvm/gems/ruby-2.6.0/extensions/x86_64-linux/2.6.0/nokogiri-1.10.1/mkmf.log
extconf failed, exit code 1
Gem files will remain installed in /home/globewalldesk/.rvm/gems/ruby-2.6.0/gems/nokogiri-1.10.1 for inspection.
Results logged to /home/globewalldesk/.rvm/gems/ruby-2.6.0/extensions/x86_64-linux/2.6.0/nokogiri-1.10.1/gem_make.out
I've looked through installation instructions and many similar questions here and elsewhere, and tried a bunch of stuff, and nothing seems to work.
I get a similar error message when I try gem install nokogiri.
Help!
UPDATE: Here is the mkmf.log file:
"gcc -o conftest -I/usr/share/rvm/rubies/ruby-2.6.0/include/ruby-2.6.0/x86_64-linux -I/usr/share/rvm/rubies/ruby-2.6.0/include/ruby-2.6.0/ruby/backward -I/usr/share/rvm/rubies/ruby-2.6.0/include/ruby-2.6.0 -I. -O3 -ggdb3 -Wall -Wextra -Wdeclaration-after-statement -Wdeprecated-declarations -Wduplicated-cond -Wimplicit-function-declaration -Wimplicit-int -Wmisleading-indentation -Wpointer-arith -Wrestrict -Wwrite-strings -Wimplicit-fallthrough=0 -Wmissing-noreturn -Wno-cast-function-type -Wno-constant-logical-operand -Wno-long-long -Wno-missing-field-initializers -Wno-overlength-strings -Wno-packed-bitfield-compat -Wno-parentheses-equality -Wno-self-assign -Wno-tautological-compare -Wno-unused-parameter -Wno-unused-value -Wsuggest-attribute=format -Wsuggest-attribute=noreturn -Wunused-variable -fPIC conftest.c -L. -L/usr/share/rvm/rubies/ruby-2.6.0/lib -Wl,-rpath,/usr/share/rvm/rubies/ruby-2.6.0/lib -L. -fstack-protector-strong -rdynamic -Wl,-export-dynamic -Wl,-rpath,/usr/share/rvm/rubies/ruby-2.6.0/lib -L/usr/share/rvm/rubies/ruby-2.6.0/lib -lruby -lm -lc "
checked program was:
/* begin */
1: #include "ruby.h"
2:
3: int main(int argc, char **argv)
4: {
5: return 0;
6: }
/* end */
"gcc -I/usr/share/rvm/rubies/ruby-2.6.0/include/ruby-2.6.0/x86_64-linux -I/usr/share/rvm/rubies/ruby-2.6.0/include/ruby-2.6.0/ruby/backward -I/usr/share/rvm/rubies/ruby-2.6.0/include/ruby-2.6.0 -I. -O3 -ggdb3 -Wall -Wextra -Wdeclaration-after-statement -Wdeprecated-declarations -Wduplicated-cond -Wimplicit-function-declaration -Wimplicit-int -Wmisleading-indentation -Wpointer-arith -Wrestrict -Wwrite-strings -Wimplicit-fallthrough=0 -Wmissing-noreturn -Wno-cast-function-type -Wno-constant-logical-operand -Wno-long-long -Wno-missing-field-initializers -Wno-overlength-strings -Wno-packed-bitfield-compat -Wno-parentheses-equality -Wno-self-assign -Wno-tautological-compare -Wno-unused-parameter -Wno-unused-value -Wsuggest-attribute=format -Wsuggest-attribute=noreturn -Wunused-variable -fPIC -Werror -c conftest.c"
checked program was:
/* begin */
1: #include "ruby.h"
2:
3: int main() {return 0;}
/* end */
UPDATE 2: Taking Javier's advice, I looked at the docs for installing Nokogiri and found the command gem install nokogiri -- --use-system-libraries, which worked. Then I did the same for rails and railties.
Now I'm getting a different error message when I try to run rails new:
Traceback (most recent call last):
2: from /home/globewalldesk/.rvm/gems/ruby-2.6.0/bin/rails:23:in `<main>'
1: from /usr/share/rvm/rubies/ruby-2.6.0/lib/ruby/2.6.0/bundler/rubygems_integration.rb:482:in `block in replace_bin_path'
/usr/share/rvm/rubies/ruby-2.6.0/lib/ruby/2.6.0/bundler/rubygems_integration.rb:462:in `block in replace_bin_path': can't find executable rails for gem railties. railties is not currently included in the bundle, perhaps you meant to add it to your Gemfile? (Gem::Exception)
O
UPDATE 3:
I accepted Javier's answer. I've been working with a fresh install (attempt), so it doesn't matter if I just start completely fresh. I don't know what ultimately screwed up my system (maybe because I copied an old .bashrc file from a different system without first removing the rvm lines?), but completely uninstalling all Ruby, Rails, other gems, and rvm, and reinstalling should fix the problem.
UPDATE THE LAST:
It took quite a bit to completely uninstall Ruby, Rails, and especially RVM. I didn’t realize how important the Bash profile scripts are, or that the lines about RVM need to be thoroughly removed from them, or how important the state of certain environment variables set by RVM are. I think the main reason I was having trouble installing Rails was simply about I had copied RVM settings from my old bash profiles. I should have removed them.
Everything is working as expected now.

Answer 1

Here are the docs to install nokogiri https://nokogiri.org/tutorials/installing_nokogiri.html, there is a note for rvm:
Note for RVM users: you may require libgmp, consider running sudo
apt-get install libgmp-dev.
Remove ruby with rvm https://rvm.io/rubies/removing
rvm remove 2.6.0
Install
Ruby
rvm install 2.6.0
Rails
gem install rails
For a existing project
bundle install