I can't run 'cap XXX deploy' to remote host - ruby-on-rails

Question

I can ssh login to my AWS's host, But when I want to use Capistrano for deploying something to a remote host, it says
NoMethodError: undefined method `gsub' for nil:NilClass
ruby version:ruby 2.3.3p222 (2016-11-21 revision 56859) [universal.x86_64-darwin17]
It's my error log trace:
cap demo deploy --trace
** Invoke demo (first_time)
** Execute demo
** Invoke load:defaults (first_time)
** Execute load:defaults
cap aborted!
NoMethodError: undefined method `gsub' for nil:NilClass
/Library/Ruby/Gems/2.3.0/gems/capistrano-3.11.0/lib/capistrano/defaults.rb:2:in `block in <top (required)>'
/Library/Ruby/Gems/2.3.0/gems/capistrano-3.11.0/lib/capistrano/configuration/validated_variables.rb:89:in `block in assert_valid_now'
/Library/Ruby/Gems/2.3.0/gems/capistrano-3.11.0/lib/capistrano/configuration/validated_variables.rb:88:in `each'
/Library/Ruby/Gems/2.3.0/gems/capistrano-3.11.0/lib/capistrano/configuration/validated_variables.rb:88:in `assert_valid_now'
/Library/Ruby/Gems/2.3.0/gems/capistrano-3.11.0/lib/capistrano/configuration/validated_variables.rb:44:in `set'
config/deploy.rb:1:in `<top (required)>'
/Library/Ruby/Gems/2.3.0/gems/capistrano-3.11.0/lib/capistrano/setup.rb:27:in `load'
/Library/Ruby/Gems/2.3.0/gems/capistrano-3.11.0/lib/capistrano/setup.rb:27:in `block (3 levels) in <top (required)>'
/Library/Ruby/Gems/2.3.0/gems/capistrano-3.11.0/lib/capistrano/configuration/variables.rb:32:in `untrusted!'
/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0/delegate.rb:83:in `method_missing'
/Library/Ruby/Gems/2.3.0/gems/capistrano-3.11.0/lib/capistrano/setup.rb:26:in `block (2 levels) in <top (required)>'
/Library/Ruby/Gems/2.3.0/gems/rake-10.4.2/lib/rake/task.rb:240:in `block in execute'
/Library/Ruby/Gems/2.3.0/gems/rake-10.4.2/lib/rake/task.rb:235:in `each'
/Library/Ruby/Gems/2.3.0/gems/rake-10.4.2/lib/rake/task.rb:235:in `execute'
/Library/Ruby/Gems/2.3.0/gems/rake-10.4.2/lib/rake/task.rb:179:in `block in invoke_with_call_chain'
/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0/monitor.rb:214:in `mon_synchronize'
/Library/Ruby/Gems/2.3.0/gems/rake-10.4.2/lib/rake/task.rb:172:in `invoke_with_call_chain'
/Library/Ruby/Gems/2.3.0/gems/rake-10.4.2/lib/rake/task.rb:165:in `invoke'
/Library/Ruby/Gems/2.3.0/gems/rake-10.4.2/lib/rake/application.rb:150:in `invoke_task'
/Library/Ruby/Gems/2.3.0/gems/rake-10.4.2/lib/rake/application.rb:106:in `block (2 levels) in top_level'
/Library/Ruby/Gems/2.3.0/gems/rake-10.4.2/lib/rake/application.rb:106:in `each'
/Library/Ruby/Gems/2.3.0/gems/rake-10.4.2/lib/rake/application.rb:106:in `block in top_level'
/Library/Ruby/Gems/2.3.0/gems/rake-10.4.2/lib/rake/application.rb:115:in `run_with_threads'
/Library/Ruby/Gems/2.3.0/gems/rake-10.4.2/lib/rake/application.rb:100:in `top_level'
/Library/Ruby/Gems/2.3.0/gems/rake-10.4.2/lib/rake/application.rb:78:in `block in run'
/Library/Ruby/Gems/2.3.0/gems/rake-10.4.2/lib/rake/application.rb:176:in `standard_exception_handling'
/Library/Ruby/Gems/2.3.0/gems/rake-10.4.2/lib/rake/application.rb:75:in `run'
/Library/Ruby/Gems/2.3.0/gems/capistrano-3.11.0/lib/capistrano/application.rb:14:in `run'
/Library/Ruby/Gems/2.3.0/gems/capistrano-3.11.0/bin/cap:3:in `<top (required)>'
/usr/local/bin/cap:22:in `load'
/usr/local/bin/cap:22:in `<main>'
Tasks: TOP => demo
THX Have a good day.

Answer 1

As far as I can tell, from a few minutes of looking in source-code, this comes from the lack of :application setting in either config/deploy.rb or the environment specific deploy-config file.
We have, for example, just an excerpt of config:
set :application, 'myappname'
set :repo_url, 'git#github.com:org/app.git'
set :deploy_via, :remote_cache
# Default branch is :master
set :branch, :master
The error is thrown, from validate :application do |_key, value| in capistrano, which suggests that application is nil - Which it will be, if the value is unset (see config above).
Can you validate, that you have set :application in the config?