Hello, certain features are only available for higher tiered plans that want to accomplish more capability than the basic plan. When you are setting up your app, certain API's may be restricted as they would not be available on the lower tier plans.

When you create your app, in Merchant Availability you can change what all your app will be able to accomplish and you should see it update which merchants it is able to function on depending on what capabilities that you select.

In the attached images I selected Discounts which is not available on our Payments Plus plan.

So your app must have some capability that is mnot compatible with the payment plan. Please note that sandbox payment plans should not have real world affect so the payment plan on the sandbox environment can be tested and changed without real world implications