You don't have permission to purchase subscriptions on this iPad Pro

Is this a corporate or school iPad? If so, subscription purchases may be restricted by the management profile.

If its a child iPad on Family Sharing, only subscriptions shared by the organizer may be permitted in Settings->Screen Time->Content & Privacy Restrictions->iTuns & App Store purchases->In-App Purchases.

If you have the screen time passcode you may set the above to "Allow", to permit subscriptions.

O.k, then check Screen Time Settings, and make sure in-app purchases are allowed as explained above.

You may have turned that on yourself, even if its not a child's iPad.