Hi Graham.
There was an unexpected response from HMRC when the pre-finalised tax calculation was returned with error messages that meant the tax return couldn't be finalised, resulting in zero amounts displayed. This was resolved yesterday afternoon. Your tax return dashboard will now show your correct amounts and when you click on the tax calculation title or amount, you will see the error messages from HMRC that are blocking the finalisation.
To add your period of account, click on your self employment income total from the tax return dashboard and enter the period at the bottom of the income source disclosures page. You will then be able to finalise the return, assuming HMRC return no further errors.
Further guidance can be found here