Q:

Bundler could not find compatible versions for gem "bundler": In Gemfile:

I'm making GitHub blog page with Jekyll. I've failed like numerous times and just forked new one and followed README on the repository. 
It's https://github.com/janczizikow/sleek/blob/master/README.md
and I tried to run  Inside the directory(master) but this msg keeps coming out. 
I tried 'gem install bundler' 'gem cleanup' and similar solutions on the internet but still can't fix it. I get that I need another version which is lower one, then how can I remove the current one and install the version I need?
I've never used Ruby so I'm SO lost. A little advice will mean a lot.
bundle install
Fetching gem metadata from https://rubygems.org/...........
Fetching gem metadata from https://rubygems.org/.
Resolving dependencies...
Bundler could not find compatible versions for gem "bundler":
  In Gemfile:
    bundler (~> 1.12) x64-mingw32

  Current Bundler version:
    bundler (2.0.2)
This Gemfile requires a different version of Bundler.
Perhaps you need to update Bundler by running `gem install bundler`?

Could not find gem 'bundler (~> 1.12)' in any of the relevant sources:
  the local ruby installation

A:

Version 2.0.2 comes from gem install bundler, but the project uses bundler version 1.12. 
To fix your issue run  

gem install bundler -v 1.12 
bundle _1.12_ install 

To start Jekyll run jekyll serve or bundle exec jekyll serve.

