Let's say I have a prepaid card with a balance of 0$ and I am subscribed to a Patreon user for the recurring monthly donation of $50. Let's say I remain subscribed to the Patreon user for 10 months, do I have the contractual obligation to pay the user if it's supposed to be a donation? How does the law define contractual obligation and can Patreon sue me for the money that owed? If Patreon does not have the right to sue, can the Patreon user sue me or Patreon for the money owed?
Why do you think this is a “donation”?
It is clear from Patreon’s ToS that you are entering into a contract with them for which you pay and they (through their creators) provide a service. You can cancel at any time but until you do you have to pay.