I was facing a strange issue In source tree. I spent about 5 hours and scratching my head. I created a repository on GitHub and clone in source tree. But whenever I tried to push code on server, it always return 403 (The requested URL returned error: 403, creditinial not found )Here I resolved that issue

  1. Create PAT token on GitHub
  2. Give all access to that PAT token like upload on repo and user name
  3. In source tree, login from that user
  4. In password , paste that token

Then push to server. it should work fine and dont fotget to make user account as default if you are using multiple account.