You might get below error if you have multiple versions of bundler installed and the default one is not the one required in your project.
The Problem
Warning: the running version of Bundler (2.1.2) is older than the version that created the lockfile (2.1.4). We suggest you to upgrade to the version that created the lockfile by running `gem install bundler:2.1.4`.
The Fix
$ gem uninstall bundler
$ gem install bundler --default
$ gem list bundler
> bundler (default: 2.1.2, default: 2.1.4)
$ rm versions/2.7.0/lib/ruby/gems/2.7.0/specifications/default/bundler-2.1.2.gemspec