The E66 error in Brother Embroidery Machine indicates that the motor has failed to rotate. Here are some possible causes and solutions for this error:

1. Thread jam or tangle: Check for any thread jams or tangles in the machine, especially around the bobbin area, and clear them if found.
2. Obstruction in the needle bar: Check for any obstructions or debris in the needle bar area and remove them.
3. Faulty motor: The motor may be faulty and need to be replaced. Contact a Brother authorized service center for assistance.
4. Loose or disconnected wiring: Check the wiring connections of the motor and make sure they are securely connected. If any wiring is damaged or disconnected, repair or replace it.
5. Mechanical issue: A mechanical issue such as a broken gear or belt can also cause the motor to fail to rotate. In this case, contact a Brother authorized service center for assistance.

It is recommended to follow the troubleshooting steps mentioned in the user manual or contact a Brother authorized service center for assistance.