@seinper our payment processor (stripe) controls whether a card is accepted or not. All handling of cc info is handled on their site for security reasons so this is not something we have control over. We’ve had a couple complaints about this in the past, and I’m not not sure why certain cards are being rejected.
You might try using a different card?
If this doesn’t work and you’re willing to commit to a year of Plus (as opposed to month to month) I can send you a paypal invoice and add you manually. Send us an email at support at hooktheory dot com and we’ll set you up.