Error: Empty addresses in Advancements/events

Hello! I keep getting “Error: Empty addresses” when I save an event in Internet Advancement . There’s about 170 recipients of this event.
I set the reminders to be 1 day before, 3 days before, 1 week before, and 1 ASAP. The hit “Save”, and after about 60 seconds of loading, it get this error.

@DavidPowers1 so I assume you enter in everything and Click Create (or Save whatever it says) and get the error?