not 100% sure on this so take this with a grain of salt, pretty sure you can only pay using paypal on NSFW/18+ games since paypal is only for people over 18 (therefore easier to tell if you're allowed to be buying the game or not!)
hii i had this issue and for me where i live we don't have paypal so i cant pay with that form so thats very unfortunate i've been a fan of this game for a year and a half now