Hi Maris 182,
welcome to the Apple (user-to-user) Community!
Re: "Why is payment method allowed for subscriptions only" :
"I don’t want to add another card I just want to use my card".
As we are all Apple users, like yourself, in the community, we have no access to your accounts or detsils in order to check the reason out for you ... generally, we do not know why Apple does what it does. We'd like to help, however
Maybe your card is only listed in Wallet for Apple Pay to merchants etc ?
Wondering if that payment method is also listed ok in your Apple ID account.
(ie: the one account for all purchases from / payments to Apple.)
Eg: You could log-in online to check, and also check that the expiry date is ok:
Apple ID Log-in:
https://appleid.apple.com/
If needed: How to: Get a verification code and sign in with two-factor authentication - Apple Support
Else: You might want to reach out to the App Store Support staff for an explanation:
They have a website online (link below) with a "Get Support - start now" link at the bottom:
You can likely choose from online chat / phone / email etc.)
App Store - Official Apple Support
All the best :-)