Check you app console.
If you go to In App purchases on the left, the screen should look like the one below.
If not, then you must have created some in app purchases at some time. Disable or remove them.
Do the same check on subscriptions as well.
Thank you both for your replies. Andrew, my purchases and subscriptions screens look just like that. Filippo, yeah, they show and say nothing else. Here's the complete message: