I'm trying to buy the game via PayPal since yesterday, but whenever I try I get an error after logging in to PayPal saying "Unable to process the transaction. Please try again later"

Anyone know why is that?

Can't help you with that I'm afraid, payment processing is all on the itch.io side. Looks like several others have been able to buy the game today, so I guess, try again later? ¯\_(ツ)_/¯