Product: Programmable SMS
OptimizeEndAt time must exceed OptimizeStartAt and be at within 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 OptimizeEndAt parameter is using "[YYYY]-[MM]-[DD]T[HH]:[MM]:[SS]Z" format (in UTC).
Check if OptimizeEndAt parameter exceeds (is after) the OptimizeStartAt parameter.