I can't buy the student version for the app Liquid text

I called the support team from Apple and from the app and haven't solve the problem, what coul it be? The screen just shows me that there is something wrond at the final moment of the purchase but everything seems fine until that final moment and it recommends me to talk with apple support (is a purchase within the app, not at the apple store)

I was on family sharing but not anymore so I could purchase this app and others with my own debit card, it has work with other apps (not purchases within the apps) but not with this one.