Unable to purchase PlayStation gift card with funds in Apple Account

.... I wanted to buy a gift card but I can’t although I have funds in my account.

Apple does not permit the purchase of gifts of any kind with your Apple Account credit balance (a/k/a "funds in your account".) You need to have a valid credit card on file.

I have funds in my account.

What type of account is that? Bank checking account? Credit card? PayPal? Apple Account balance? Apple Cash?

Where did you want to buy this? Apple doesn’t sell PlayStation Gift Cards, but Sony does. Apple sells Apple Gift Cards, both physical and electronic, usable for Apple products and services (including in-app purchases).

To see what you can use your Apple Account funds for, review the information provided here:

What you can buy with your Apple Gift Card or Apple Account balance - Apple Support