Once you convert, you can no longer cancel the match, and can't convert it back to private. You would be on the hook to pay if the slot doesn't fill. Although maybe you're already past that point.
https://playerhelp.playtomic.com/hc/en-gb/articles/39545442764945-How-to-convert-a-Private-Booking-to-a-Public-Match
Yeah already beyond refund point which is why I don’t mind paying for someone else to play me