A similar issue is described here however, my symptoms are a little different and none of the steps there seemed to work. Nor does the suggestion here work.
I\'m attemp
I had same problem. My fix way:
Remove user from SourceTree settings (optional, i not sure);
Add you account in setting and generate new SSH key (it's a main part of fix);
Clone your repo;
If SourceTree sill can't clone repo you can try to clone from main page of SourceTree choosing external repos.
It happens because SourceTree didn't get some private access from Github while authenticating. So the solution is very simple
Fill Note as sourcetree
and Check All Scopes from checkbox as show in below screenshots
After Click on Generate Token
Now Open sourceTree
Click on sourceTree preference & Click add Account
Hope it helps
I managed to solve this by adding the remote private repo by cloning with SSH