To explain the problem, your error is as follows:
LoadError: cannot load such file -- uglifier
(in /home/cool_tech/cool_tech/app/assets/javascripts/application.js)
This means somewhere in application.js, your app is referencing uglifier (probably in the manifest area at the top of the file). To fix the issue, you either need to remove the reference to uglifier, or make sure the uglifier file is present in your app, hence the answers you've been provided
Fix
If you've had no luck with adding the gem to your GemFile, a quick fix would be to remove any reference to uglifier in your application.js manifest. This, of course, will be temporary, but will at least allow you to precompile your assets