I am sorry that you are receiving this error when trying to link your project to your Github. So we can investigate further, at what step in the linking process are you given this error?
This often happens when you have more than one glitch account, and you are logged into the account that is not the one you linked to your github account.
In addition, users can sometimes accidentally create a new Glitch account if they try to sign in using a method that is different from when they initially used.
Is it possible that you may have more than one Glitch account?