Hello @MikeR503 ,
Does this message only appear when scheduling a Resend to Non-Openers? Or does it occur when just trying to normally schedule a new email to a list? Does it occur with a fresh, new email, or one that was copied from another? Does the issue occur in other browsers, or in Incognito/Private browsing?
Does this issue seem to occur when the "Send early results" option is checkmarked on?