35111: SendAt timestamp is missing

This error occurs when you try to create a scheduled message with ScheduleType set to fixed and do not include SendAt. Scheduled messages require both parameters, and SendAt must be a timestamp in ISO-8601 format.

• You set ScheduleType to fixed but left out SendAt.
• You tried to schedule a message without providing the send time Twilio needs to create the scheduled Message resource.

• Include SendAt whenever you set ScheduleType to fixed. Use an ISO-8601 timestamp.
• Make sure the scheduled send time is between 15 minutes and 35 days from the time Twilio receives the POST request.
• Create the message with a MessagingServiceSid when you schedule delivery. Without it, Twilio treats the request as non-scheduled.

• Message Scheduling
• Messaging API Overview