Problem with ArduSub checkout

Hi, I am trying to build a custom frame in ArduSub using this guide Guide . But when I try to checkout to ArduSub-stable, it says the following “fatal: ‘ArduSub-stable’ is not a commit and a branch ‘new-branch’ cannot be created from it”. I have also tried it manually by getting the commit code from github but then it says " fatal: reference is not a tree: f2af3c7ed2907be914c41d8512654a77498d3870". I have read other forum posts but can’t get it to work. Any help would be greatly appreciated!
Thanks in advance,
Sid