Sprwt does not allow two group campaigns to be scheduled for exactly the same date and minute. If another broadcast (one-off or the first send of a recurring campaign) already uses that timestamp you get "We Already have a message scheduled at <time>, Please consider scheduling your message in future time". Pick a different minute and confirm again. A similar check rejects times that are already in the past ("We can't send your message in <time>, Please consider scheduling your message in a future time").