This very blog runs on Jekll (:-D) and while running the local server via
$ bundle exec jekyll serve I got the error
zsh: command not found: bundle.
After a quick Google search, I reinstalled the bundler ruby gem via
$ gem install bundler.
Trying to run the Jekyll serve command again, I got an error from ruby that bundler required an update. So, following the directions from the output i did
$ bundle update --bundler.
That resolved the issue and I was able to run my Jekyll server normally.