The full error message reads:
Government Gateway log-in failure, please check your User ID, UTR and Gateway password are correct. You can check your User ID and password are valid by logging into the Inland Revenue portal at https://online.hmrc.gov.uk/home. If you view your statement after logging in you will also see confirmation of your UTR.
This error message is always correct and is caused by an error in the entry of the User ID, Password, or UTR on the front page of the Ftax form. Common mistakes include entering an inappropriate UTR, e.g. the Partnership UTR instead of the Individual UTR. Characters are sometimes entered incorrectly, e.g. an ‘l’ can be entered instead of an ‘1’, or an ‘S’ entered instead of a ‘5’. Please check very carefully. Also check that your Password has been activated using the PIN sent to you in the post by Government Gateway when you first registered for online filing.