There are many reasons why your device is not recognized on the Microsoft Store. Assuming you are using the same Microsoft account on the Store and the PC, some of the possible causes and resolutions are as below:
1. You bought an app/game that is not intended for PC.: For Example, Minecraft has 2 versions on the Microsoft Store. One is for PC, and other is for Xbox. Make sure you did not buy the Xbox one - if so, take a refund immediately and buy again. Xbox version requires game pass to run on PC.
2. Your device is not up to date: Many games or apps require supported release of Windows to run. Make sure your Windows version is up-to-date. A 2-year-old OS release may have compatibility issues with the app. Minimum OS requirements are listed on the app page. You can update your device from https://aka.ms/Windows10
- Your PC hardware does not meet the minimum app/game requirements: Certain apps/games, like Forza Horizon 4 require high-end graphics to run, especially a dedicated GPU. If your device does not have one, you may not be able to install the app. In that case, consider updating hardware.
- Make sure you are logged in to the Xbox console companion app: Xbox console companion app in Windows helps to track your achievements in the game. If you are not logged in there, your device may not be recognized. In that case, open it and log in using the same account. You can also try installing the app or game from the Microsoft Store in built the Xbox Console companion on the left menu.
- If you logged in using a Microsoft account recently/did a clean install: If you recently logged into your Windows 10 device recently, it takes a while to propagate the change to the devices list. In that case, wait for some hours, likely 12 and try again.
- The device limit is exceeded: Microsoft Store allows only 10 devices/account to install an app or game. If you think you are exceeding the limit, remove devices from https://account.microsoft.com/devices and try again.