There is a soft limit of 500,000 scheduled messages at any given time, per account_sid, that can be guaranteed. As each scheduled message is sent, more messages can be scheduled up to that limit.
The account_sid already has at least 500,000 messages scheduled.
Wait for the scheduled messages to be sent. At the send_at time, as the messages are sent (or canceled) the limit will free up. If this limitation is a blocker, kindly contact Twilio Support through the Console or Help Center.