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.
Delete the github.com password from the Keychain Access.
Delete the credentials from Credential Manager.