You can encounter this error even if you believe that you have the appstore configured correctly. But this is the first thing to check becuase this error will be thrown if this is not configured correctly. You can install and configure it by following the instructions at:
Following, ensure that the App domain and App prefix are correct through central administration. At this point, it also makes ense to ensure that the language and currency are correct, since this error can be thrown when hitting a localization error. If at this point, you aren’t having a ton of success and still encountering the error, consider recreating the Subscription Settings Service Application.
If that doesn’t work, stoke a fire under the WFE.