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I noticed that you are using the code
_storeContext = StoreContext.GetForUser(firstUser);
This is the code for multi user apps. As @Viorel said, if you are just a normal UWP application which is single user, you could just try the following instead
_storeContext = StoreContext.GetDefault();
Then you could call the following code:
StoreRateAndReviewResult result = await _storeContext.RequestRateAndReviewAppAsync();
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Any updates about this?