There are a couple of reasons why you might not be able to purchase a Projector Subscription.
iCloud Drive is not enabled on your device
The Apple iCloud Drive feature must be enabled on the device that you are using in order for Projector to purchase a subscription. The following Apple help article gives instructions on how to enable iCloud Drive on your device:
If you do not have iCloud Drive enabled on your device, the Projector Trial Expired screen will not have the Subscribe Now button present, nor will the Projector Subscription Expired screen have the Subscribe Again button. Each screen will have explanatory message indicating that you must enable iCloud Drive on your device in order to purchase a Projector Subscription:
Projector will logout when the screen is dismissed with the OK button.
Solution
You can setup iCloud Drive on your device, restart Projector, and continue to purchase a Projector Subscription if you so desire.
Your Apple ID owns a Projector Subscription for another organization
The Projector Subscription can only be associated with a single Planning Center Services organization. If your Apple ID has already purchased a Projector Subscription for an organization and the subscription is still active, you cannot purchase a separate Projector Subscription for another organization on that Apple ID.
If this is the case, the Projector Trial Expired screen will not have the Subscribe Now button present, nor will the Projector Subscription Expired screen have the Subscribe Again button. Each screen will have explanatory message indicating that your Apple ID has already purchased a Projector Subscription, and will name the organization that it was purchased for:
Projector will logout when the screen is dismissed with the OK button.
Solution
You will need to run Projector on a device that is signed into a different Apple ID if you want to purchase a Projector Subscription for the new organization.