Can not add redeem key on Sandbox environment.

Question

Hi, I'm facing with an issue about redeem code on Sandbox environment. I'm working on a project, an I've generated some key of my application for redeem to test. When I redeemed the code on my Sandbox account on Xbox app or Microsoft store, the store could detect my app, but when I click confirm to add, the error message "Try that again, Something happened at our end, waiting a bit might help." is shown. So, this is the Sandbox problem? Or, how can I fix it?