If you're getting an authentication error when trying to make a submission using the HMRC Basic PAYE Tools then check your password length. A password of more than 12 characters will not work.
The website lets you set up a longer password and use it to log in but it will not work for the Basic PAYE Tools.
Change your password on the website to 12 characters or less and you should be able to make your submission. Note that on the password change page it does tell you it should be 12 characters or less.
Thank you!