find_spec_for_exe’: can’t find gem bundler (>= 0.a) (Gem::GemNotFoundException)

The problem in my case is that the Gemfile.lock file had a BUNDLED_WITH version of 1.16.1 and gem install bundler installed version 2.0.1, so there was a version mismatch when looking to right the folder

gem install bundler -v 1.16.1 fixed it

Of course, you can also change your Gemfile.lock‘s BUNDLED_WITH with last bundler version and use recent software, as Sam3000 suggests