SA100 Authentication Error

In order to submit Self Assessment electronically, your government gateway ID must be enrolled for the HMRC Self Assessment service - which involves requesting it to be switched on, and then receiving an activation code in the post from HMRC, which you must type in to the HMRC website. If you do not complete that process, you will be able to sign in to the HMRC website but any attempt to submit will be rejected with an Authentication Error.

To see if Self Assessment has been activated for your Gateway ID, please sign in to the HMRC website using that gateway ID and password, then look for a 'Self Assessment' section. If that displays instructions on how to enrol for Self Assessment, please follow the link and complete the form. That will trigger HMRC to put an activation code in the post.

If Self Assessment is activated and you are receiving Authentication Error please do the following:

1. Check that you are doing a return for an Individual not a Partnership (SA100 is for an individual, though that individual may contain income received from one or more partnerships). For returns for a Partnership itself, the SA800 should be used.
2. Use the 'Show' button in the 'Submit' popup to look at the password you have entered
3. Check that UTR number you have entered in 'About you' is correct
4. Sign in to the HMRC website https://www.gov.uk/log-in-register-hmrc-online-services using the credentials you are entering in to ABC SA800 Partnership Tax Return
5. Look for a 'Self Assessment' section; if that displays instructions on how to enrol for Self Assessment, follow the link and instructions, which will trigger HMRC to put an activation code in the post. When that arrives, follow the instructions to complete activation.

Finally, some users find that, without realising it, they have multiple Self Assessment username/passwords and are using the wrong one. This only comes to light from talking to the HMRC online Self Assessment helpline.