Why the theme can't be found when being built in github?

[TravisBAO]

I use the Gem_Based method to install the theme, and everything is well locally. I could see it with the test command "bundle exec jekyll serve".

But When I push it to github, the build is failed with the message like that:
github-pages 227 | Error: The minimal-mistakes-jekyll theme could not be found.

I have no idea about it. It's so strange.
Could you please see my repo for it?