Default [Play Store] "The item you were attempting to purchase could not be found"

Hello,
I hope this is the right section to post in.

I have a free game on Play Store with the ability to unlock full game by making an in-app purchase.
When users try to purchase that item, they get this error: "The item you were attempting to purchase could not be found".
The item is published and active. Some months ago there were no problems. What do you think it is the cause?
Thank you in advance!