The fix
gem update --system # update Rubygems
gem install bundler # update bundler
bundler update --bundler # update Gemfile.lock in your project
The error message
>bundle exec guard
14:41:40 - INFO - Guard::RSpec is running
14:41:40 - INFO - Guard is now watching at '/Users/user/project'
14:41:52 - INFO - Running: spec/spec.rb
You must use Bundler 2 or greater with this lockfile.
14:41:52 - ERROR - Failed: "bundle exec bin/rspec -f progress -r /Users/user/project/vendor/bundle/ruby/2.6.0/gems/guard-rspec-4.7.3/lib/guard/rspec_formatter.rb -f Guard::RSpecFormatter --failure-exit-code 2 spec/spec.rb" (exit code: 20)
turnspike 