The following are possible reasons why you may not be able to subscribe to Feh Pass.
1. You are already subscribed to Feh Pass.
You can only have one Feh Pass subscription per Apple ID per save data.
If multiple family members are using the same Apple ID and one of them has already subscribed to Feh Pass, other family members will not be able to subscribe to Feh Pass.
In this situation, another Apple ID would need to be used.
If a Feh Pass subscription has been canceled but has not expired yet, Feh Pass cannot be subscribed to again until the existing subscription has expired.
2. You are already subscribed to Feh Pass on another platform.
If you are using multiple devices with different operating systems to access the same save data, you may only be subscribed to Feh Pass through one of them.
3. Purchasing is restricted.
If credit card usage is restricted or parental controls that restrict making purchases have been implemented, then a purchase cannot be made.
Please also confirm that there are no other issues with the payment method being used.