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Presumably you have done this:
https://www.gov.uk/log-in-file-self-assessment-tax-returnIf you’re filing online for the first time
Before you can sign in you must:have your UTR – you’ll find your UTR on the registration letter HMRC sent you
enrol for the online service (you can do this when you create a Government Gateway account)
activate the service using the code you’ll be sent in the postCall them and say you are having problems logging in to the online service. Nothing to do with forms in the post or anything. Number here
https://www.gov.uk/government/organisations/hm-revenue-customs/contact/self-assessment-online-services-helpdesk(It's a different number to advice on self-assessment. They are open today.)
hoefla
I enter my UserID, my password, it prompts for the code from the SMS, I enter it, we go back to the above page again.
I enter my UserID, my password, screen flashes and clears both boxes.