I have same issue , and when I contacted Apple Search Ads they said They don’t accept prepaid card . I use same Prepaid Visa Card on search ads for years , and now I got renewal card but I can’t replace old one and save it . I use this card every where without issue even to buy ios apps from Apple Apps Store , but can’t use it for search ads !

still I didn’t get financial report for April 2021 . I searched on Google and found this post