Why does my account says "You're out of space"

Another thing, why do we get the "out of space" message? The form has only been used 4 times, 2 for tests and the other 2 for real.

You're already out of space because the Free Plan only allows 1 payment per month.

You can also confirm this on your Account's Usage page. All monthly limits replenish on the 1st day of each month so you can either wait for this limit to reset back to 0 or upgrade to a paid plan that allows more payments per month.