I recently activated two-factor authentication on GitHub, but that started causing an error in SourceTree:

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What that means is that you can no longer use your regular account password to login to GitHub from SourceTree, but have to create a token instead.

You can do this on the application settings page by clicking on the Create new token button and entering a name for the application (e.g. SourceTree). Then copy the token by clicking on the clipboard icon:

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Lastly, simply use that as your user’s password in SourceTree. The error message should now go away and you’re all set.