All of your prices can be viewed and updated from the Pricing page. There you'll find collapsable rows for each Site you've created and your Insurance Cover Levels.
Within each, there's a table showing prices for every active Billing Period. If a price hasn't been created, it will be displayed as NOT SET. You should create these prices, otherwise, you won't be able to create Subscriptions from the Backoffice for that item with your selected periods. In addition, these items will be hidden on the Storefront if no price is entered for them for the Storefront Billing Period.
How to set Billing Period prices
At the end of each row, there is an Edit Prices button. Press this to be taken to a form where you can set all prices across Billing Periods for the selected item.