There is a limit on how many concurrent installations you can have of the Windows version of MobileSheets. If you've installed the app on too many devices, you have to unregister some of the devices on your account. In general, you should only use this to install the app on tablets you own that will be used by yourself or family members. You cannot use this approach to install the app on devices that will be used by other musicians as this is a breach of the terms of service for the Microsoft Store.