Buffer limits free accounts to a maximum of 10 messages in the queue. When a scheduled message is scheduled to be sent, it will be sent to Buffer at 12:00 AM that day. These all count towards your queue limit.
If your Buffer queue is already full, Buffer will refuse to accept the message from CoSchedule, causing a message failure. To correct this, you can manage your queue in Buffer or upgrade your Buffer account to increase the maximum number of messages that can be in your queue at one time.