This is causing all of my in-game purchases to error, please fix this so that I can continue to earn player points for my game!
(It happened when trying to purchase a Game Pass using this method, if that matters at all.)
When prompting a product purchase you have to use the product id. The product id is not the asset id. You can find the product id using GetProductInfo on your asset id.
Thank you for letting me know how to fix this error, it helps a lot! 
However, after this error has happened I am no longer able to call that method anymore from the client, and the error is not too clear. I would love it if they would just send out a reminder (not an error) saying “Asset ID not found” or something letting developers know exactly what that error means.