Sorry for the late reply, just spotted your unanswered question.
Preeferably use a PC / laptop not the pp or mobile browser, and use Chrome or Firefox, this then will give you more options.
If there is no delivery service selected under Domestic Postage click on Add Primary Service, from the pop up screen which appears, you can find Royal Mail Tracked 48.
If there is a service already selected, click on the 3 vertical dots alongside, and click on Delete , Add Primary service will then appear again.
@suflo1970