Rails Couldn't Find Gem

Apologies in advance if this is in the wrong section, or if this is even something that should be asked, but I just started to try, do and learn rails by myself, and I always encountered the problem of not having a particular Gem, at first it was ’rails’ then ’puma’ and now ’sass-rails’. If I keep going I’m almost certain the next Gem problem I’m going to have is ’uglifier’.

So far I just had to replace the versions in the Gemlock file with the ones from here and that solved the issue for ’rails’ and ’puma’, however for ’sass-rails’ it doesn’t seem to work.

This is the error and the Gemlock file:
Server Rails Prob 3 - Merge

I don’t know what else I should be showing, or should be knowing, but I’ll do my best to find out if you need other information.

Are you using windows?