Product: Programmable SMS
OptimizeStartAt time must be between 3600 seconds and 604800 seconds in the future, inclusive.
Scheduler currently doesn't support scheduling messages for an optimized time less that 1 hour from now, or more than 7 days in the future.
Verify that the OptimizeStartAt parameter is using "[YYYY]-[MM]-[DD]T[HH]:[MM]:[SS]Z" format ( in UTC ).
Check if OptimizeStartAt parameter exceeds at least 1 hour (3600 seconds) from the time of request.