Are you in developer mode? If not, enable "Developer mode" and try again. It's at the top of the chrome://extensions/ tab.
Doing some more research, it looks like Chrome started doing this awhile ago to prevent malware, supposedly. You can now only get extensions through the Chrome web store.
Try this as a workaround:
- clone the repo
- go to chrome://extensions/
- enable "Developer mode"
- Click on "Load unpacked extension..." and select the cloned directory
I'm not asking you to publish it on the store just for me. I think the store is bullshit. They charge money from publishers, let in tons of malware, have automatic updates, and don't let people inspect the source.