I have SourceTree with local working copy. And all operations work good, I can simple fetch, push, pull and etc via SourceTree. I just needed to make force push which does not exist in SourceTree.
I opened terminal made git push -f
remote: Repository not found.
fatal: repository 'https://github.com/MyRepo/project.git/' not found
I am not sure what can be an issue.
Remove the all github.com credential details from the system.
For mac
Delete the github.com password from the Keychain Access.
For window
Delete the credentials from Credential Manager.