SA800 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 for the partnership - and those details will be associated with a specific UTR (Unique Tax Reference), which must be the UTR you enter in the 'Partnership information' section at the start of the form.
1. To submit an SA800 you should use a gateway ID specifically set up for the partnership, not the ID you use to submit as an individual
2. 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. Use the 'Show' button in the 'Submit' popup to look at the password you have entered
2. Check that UTR number you have entered in 'Partnership information' is the correct one for the partnership
3. 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
4. Once logged in, check that the UTR displayed matches the one you are using
5. Look for a Self Assessment heading and check that it doesn't display links about how to enrol for the Self Assessment service. If it does, the service is not activated and you should click the link and follow the instructions.
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 services helpdesk (note, not the General Enquiries help desk).