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
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