Not able to host app on Heroku using sqlite3 gem

Heroku Logs
Running: rake assets:precompile
rake aborted!
Gem::LoadError: Error loading the 'sqlite3' Active Record adapter. Missing a gem it depends on? sqlite3 is not part of the bundle. Add it to your Gemfile.
/tmp/build_28ce13e795827133a7d6aa8c90c74c73/vendor/bundle/ruby/2.5.0/gems/bundler-1.15.2/lib/bundler/rubygems_integration.rb:377:in `block (2 levels) in replace_gem'

Tasks: TOP => environment
(See full trace by running task with --trace)

! Precompiling assets failed.
! Push rejected, failed to compile Ruby app.

! Push failed
Any Idea?

Heroku supports postgres, so change the gem file in such a way that sqlite is used in development and gem “pg” is used in production.

not able to run server
What am I missing?
rails s

Traceback (most recent call last):

2: from /usr/local/bin/rails:23:in `<main>'

1: from /Users/apple/.rvm/rubies/ruby-2.5.1/lib/ruby/site_ruby/2.5.0/rubygems.rb:308:in `activate_bin_path'

/Users/apple/.rvm/rubies/ruby-2.5.1/lib/ruby/site_ruby/2.5.0/rubygems.rb:289:in `find_spec_for_exe’: can’t find gem railties (>= 0.a) with executable rails (Gem::GemNotFoundException)