FYI I also cannot purchase Affinity Photo with my Boeing Credit Union card. Support said they support all "debit cards", but apparently not. Tried to grab a sale over summer when I could afford it.
GIMP is a great alternative, I've used it over the years. Heck even used it at a job cause my photoshop wouldnt install under a VM, so just installed GIMP straight on the linux machine. Lol
PS this is seemingly more common, I have encountered issues with this card at a lot of smaller businesses, with whoever they are working with for purchases. Or they only take paypal which every time I use flags my account and I have got to call and verify some crap piece of information they already had.