Have added my GitHub account with OAuth. When enumerating remotes the following error continuously pops up.
The operation couldn’t be completed. (com.google.GTMOAuth2 error -1001.)
I have cleared all relevant items from macOS Keychain and and deleted ~/Library/Application Support/Sourcetree, readded my GitHub account the the issue persists.
That's an authorization error which means that Sourcetree couldn't get an authorization rom GitHub with the token available. Can you remove the account from Sourcetree and add it back again? Your old token could just be cached. Relaunch Sourcetree once to see if it helps.