SendAt time must be between 900 seconds and 35 days (3024000 seconds) in the future, inclusive.
Scheduler currently doesn't support scheduling a message at a fixed time less that 15 minutes from now, or more than 35 days in the future.
Verify that the SendAt parameter is using "[YYYY]-[MM]-[DD]T[HH]:[MM]:[SS]Z" format (in UTC).