SA100 Authentication Error
That message means that the HMRC User ID and HMRC Password you entered when submitting were rejected by the HMRC.
The user ID and password you enter need to be the same user ID and password you would use to log in to the HMRC Self Assessment website i.e the username and password the HMRC gave to you - and those details will be associated with a specific UTR (Unique Tax Reference), which must be the UTR you enter in the 'About you' section at the start of the form.
Also, the Self Assessment service must be activated for that UTR - being able to log in does not guarantee that the service is activated, and if it is not activated you will get an 'Authentication error' when you try to submit.
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. Remember that the password is case sensitive - and you must make sure you do not have any extra spaces before or after it.
4. Check that UTR number you have entered in 'About you' is correct
5. Go to the HMRC website and enter the gateway ID and password you are using to check that you can log in - that will check that the username and password is correct
6. Once logged in, check that the UTR displayed matches the one you are using
7. Click on 'Your HMRC services' and check that the Self Assessment (SA) service does not appear under the heading 'Services you can add'. If it does appear in that list, add it then try again to submit.
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.