This can happen when the timeline between attempts conflicts.
For example, in the Treatment plan follow-up campaign in the Campaigns tab, the first attempt must be set somewhere between 1 and 14 days. If you select 14 days for attempt one, then attempt two must be sent out 15 days or after. Entering a value lower than 15 results in an error message.
This is simply a fail-safe to prevent you from sending messages out of order.