I tried to purchase Minecraft Java edition through the website. When I reach the payment section, after I submit my credit card information and hit the green purchase button:

• a small in-page pop-up is shown to take me to the payment provider's site
• Payment provider is Rabobank (Netherlands)
• This payment provider requires cookies
• I have all my cookies allowed
• The payment provider keeps saying it needs cookies no matter what I do

It looks like you're not allowing this payment provider to place cookies via your website.

Some details that might be relevant, maybe not:

• I'm trying to purchase this game for my kid, he is under age
• My browser is set to allow all cookies
• I can log in normally, which also requires cookies
• I can also log into my payment provider's site regularly

I tried this on two different devices: a Windows 10 laptop using Chrome, and an android phone using Chrome. Both had the same issue.