Uncheck the box next to the "Send Later" button.
Boomerang gives the ability to specify two points in time at the bottom of the email composition page:
- Boomerang Reminder date/time
- Send Later date/time
The Boomerang Reminder date is the one that is located to the right of the red "Send Later" button and will select a time to send you a reminder email. This is enabled by the check-box to the right of the "Send Later" button, so if you want to disable it, just un-check this box:
The Send Later date and time and specified in the pop-up dialog that appears when you click the red "Send Later" button. As mentioned before, this time is a completely different thing than the reminder date. This error occurs when the email's reminder time is set-up to be before the send-time. It makes no sense to set up a reminder on an email that has not yet been sent, which is why you get the error. You could also resolve this by setting the reminder time to be at some point after the time specified in the send-later time, in the case that you want both.