Bundler 2.0.1 installed, Rails says "must use Bundler 2 or greater"?

I ran into the same issue & resolved it by downgrading with gem uninstall bundler, followed by gem install bundler -v 1.17.3 to revert back to the last pre 2 version.

I saw this across 3 separate systems using different version managers (ASDF, rbenv, rvm) and this resolved the issue.

There is some information on getting both versions to play nicely here. The root cause of this issue may have been a bug (which appears to have since been fixed)

Per the comment from @MatijsvanZuijlen below, and the bundler documentation, you should also be able to specify which version of bundler you want to use at runtime with bundle _1.17.3_ install

This helped me: gem update --system