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.
If you have completed the activation process, your Personal Tax Account on the HMRC website will contain a Self Assessment section which looks similar to the one below (although it may appear in a different position e.g. on the right - the HMRC pages display differently for different users):
If it is not activated, the Self Assessment section will instead have a link to page describing how to activate it.
If it 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. Remember that the password is case sensitive
4. Check that UTR number you have entered in 'About you' is correct
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.