Hi Matthew, I submitted the case to Wix Velo Team and received an amazing support. I am not sure this applies to you. In my case, I had created some tests events and over time the development time the registration day of the event passed by. Hence I was trying to reserve tickets for an event whose date was expired in the past hence got an error. I recommended to improve Velo API documentation as actually there is not written anywhere that (despite it logically makes a lot of sense) the API performs this check. Once I changed in the Event Dashboard the given event day to a future date, it worked. Hope that helps.