I can't do "git push heroku master" - ruby-on-rails

Question

I get error below when I hit git push heroku master.
remote: Tasks: TOP => assets:precompile
remote: (See full trace by running task with --trace)
remote: !
remote: ! Precompiling assets failed.
remote: !
remote:
remote: ! Push rejected, failed to compile Ruby app
remote:
remote: Verifying deploy....
remote:
remote: ! Push rejected to xxx
remote:
To https://git.heroku.com/xxx.git
! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/xxx.git'
My gem file is below.
source 'https://rubygems.org'
ruby '2.3.0'
gem 'rails_12factor', group: :production
gem 'bootstrap-sass'
gem 'rails', '4.2.5.1'
gem 'sass-rails', '~> 5.0'
gem 'uglifier', '>= 1.3.0'
gem 'coffee-rails', '~> 4.1.0'
gem 'jquery-rails'
gem 'turbolinks'
gem 'jbuilder', '~> 2.0'
gem 'sdoc', '~> 0.4.0', group: :doc
group :production, :staging do
gem 'unicorn'
end
group :development, :test do
gem 'capistrano'
gem 'capistrano-bundler'
gem 'capistrano-rails'
gem 'capistrano-rbenv'
end
group :production do
gem 'pg'
gem 'rails_12factor'
end
group :development, :test do
gem 'sqlite3'
end
group :development, :test do
gem 'byebug'
end
group :development do
gem 'web-console', '~> 2.0'
gem 'spring'
end
I don't know why I got rejected to push my rails app to heroku. I think my Gemfile is alright though. if someone knows solutions for this. I would really appreciate.

Answer 1

The error indicates that there are changes which are newer than commits you have in local.
To solve this:
Pull merge heroku to your local first, then try push to heroku again
Push force to heroku if you are sure that your local is the newest one!

Answer 2

I'll add onto Heiu Pham's answer and say that in Heroku, you may have problems with pushing to the master branch (if you are behind in history for example). What you can do is to start a new branch using
git checkout -b tempbranch and then push using
git push heroku tempbranch
If you are absolutely sure there are no newer refs, you can try
git push heroku -f
If this still doesn't work, check $ heroku logs for a hint to what the problem may be.