Ruby on Rails Could not find gem 'sass-rails (~> 5.0)

I used the command run server and i had this error:

Could not find gem 'sass-rails (~> 5.0) x64-mingw32' in any of the gem sources listed in your Gemfile or installed on this machine.

After i tried to run this:

gem 'sass-rails', '~> 5.0.1'

But i have uknown command error. Someone can help me? I've just finished to install sqlLite3