Whenever I get this error, I have to again do a google search. So, this post is for me not to do that again!
Could not find 'bundler' (2.1.4) required by your .Gemfile.lock. (Gem::GemNotFoundException) To update to the latest version installed on your system, run `bundle update --bundler`. To install the missing version, run `gem install bundler:2.1.4`
Unfortunately, the suggestions don’t quite work.
The correct way to fix this is to run these three commands:
gem install --default bundler gem update --system bundler update --bundler
Courtesy of StackOverflow. (The correct answer on StackOverflow didn’t quite work either!)