The Pricing page allows you to view and update all of your prices. There, you'll find collapsable rows for each Site you've created and your Insurance Coverage 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 back office for that item within 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.

At the end of each row, there is an Edit button. Press this to start editing the prices. Click Save changes when done.