Today I wanted to buy Titanium subscriptions with a Mastercard, when I click Pay it shows me that the card was declined, use a different card.

idk what to do

Try using the new payment method, not the old one. It directs you to website and you enter your card from there, the name of it "Stripe Checkout", if it doesnt works try opening an ticket from https://support.mountaininteractive.com/portal/en/newticket

doesn't work