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: https://online.hmrc.gov.uk/self-assessment/ 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.
- 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 (which we do not offer a product for 2013-14)
- Use the 'Show' button in the 'Submit' popup to look at the password you have entered
- Remember that the password is case sensitive - and you must make sure you do not have any extra spaces before or after it.
- Check that UTR number you have entered in 'About you' is correct
- Go to https://online.hmrc.gov.uk/self-assessment/ and enter the gateway ID and password you are using to check that you can log in - that will check that you are registered for the online Self Assessment service and that the username and password is correct