You must have or set up a Personal Gateway Account set up to receive your tax return before you hit the "Submit to HMRC" button in the app.

Customers submitting their tax return online for the first time will need to request an activation code, (which activates your account) HMRC will post it to your home address, it can take up to ten days to arrive.

It's very straightforward.

Follow this link: https://www.gov.uk/personal-tax-account