I will explain my answer again.
As you are a non UK resident and a non British Citizen, you will not be able to register online on HMRC in order to submit your tax return for the year Apr 2020 - 2021. The reason is that online registration requires British Passport, Irish Driving license or Payslips which you are unable to provide.
So In order to submit the return, you have two options:
Option 1: Download a tax return from the HMRC website, manually fill it in and post it to the HMRC. (I'm attaching a blank tax return & property pages 2020-21 for your assistance)
Option 2: Appoint an Agent (Letting Agent) or an Accountant to submit the return for you.
I hope the above is helpful.