Bundler: can’t find gem bundler (>= 0.a) with executable bundle (Gem::GemNotFoundException) during bundle install with gem

Bundler version 2 introduced a new feature to automatically use the version of Bundler specified in the Gemfile.lock of your project. Thus, if you have an existing Gemfile.lock with a line like this at the bottom

BUNDLED WITH
   1.17.3

Bundler will try to run with a Bundler version < 2.0. Since you just have Bundler 2.0.1 (and Rubygems >= 2.7.0) installed, this fails with this rather unhelpful error message.

To fix this, you could

• remove the lines from your Gemfile.lock and to use bundler 2.x everywhere from now on, or
• install a bundler 1.x version with gem install bundler -v '< 2.0' to use the appropriate version as specified by your Gemfile.lock.

More information about this can be found on the Bundler blog.