Hi Robert,
HMRC have got back to us and has informed us that they can see you have two self employment income sources but have only sent quarterly updates for one and hence only one shows on the calculation.
In this case, if one of them was set up in error, so you should log into your online HMRC account to close it down. If you actually do have two self employment income sources, you will need to submit Q4 update for the second one and then it should appear on your tax calculation and allow you to set the period of account and then finalise.
We can see the income source "Robert Frater" has quarterly updates. The second income source "MR R J Frater" is the one that is causing the period of account error and has no quarterly updates. If this is the duplicate, you should close this down.
Tommy