Thanks for raising this. GitHub made some protocol permission changes, and ArduSub-stable hasn’t yet had the relevant updates to fix that. I’ve raised an issue for it, but if you want/need to use it in the meantime (before it gets fixed properly) you can
clone the repo
checkout ArduSub-stable into a new branch
open the .gitmodules file and replace git:// with https://
update submodules (but not recursively)
open modules/mavlink/.gitmodules and replace git:// with https://