Actually, @Math_X, this isn’t an issue on our end so much as it is something we did not design the Buy Button for. This is unfortunately an incompatibility between how you’ve set up your data and how you’ve configured your buy screen.
Your buy screen is trying to read a “Purchase ID” from the SIGN UP sheet to indicate which product is being purchased. However, this same SIGN UP sheet uses our Row Ownership feature. So if you’ve never signed in before, and you do not have a row in this sheet, we can’t resolve a “Purchase ID” from said sheet.
We use the “Purchase ID” to determine how much to charge your customers. It needs to be able to pick out a row from your sheet in order to determine this. The way your sign up flow is currently configured, you are trying to charge a user with a “Purchase ID” that cannot exist because they do not have a row that they own yet.
One workaround you could try is to introduce yet another screen for buying access to your app. This screen needs to be bound to a sheet that everyone can read, and it needs to include some unique identifiers with a range of prices. When we write the purchases to the special sheet, you could either use formulas or Google Sheets scripts to move this information into the rows that the signed-in user owns.
We’re still trying to design a workable solution to allow for in-app subscriptions, but we can’t commit to delivering one that is both easy to use and fairly useful in the near term.