To link a YouTube account, follow this operation:
From here, you can select videos to add to your Steam profile.
First, ensure that you are entering the correct credentials for your YouTube account.
If your entered credentials are correct, this error can be caused by YouTube's two-factor authentication system. You can resolve this issue by setting up a unique password for your YouTube account. Refer to Google's documentation for more information:
Once your unique password is set up, test the issue again. Your accounts should now link without issue.