The E20 error in a Brother Embroidery Machine refers to a problem with the upper thread sensor. Here are some possible causes and solutions for this error:

1. Thread tension is too high: Check the thread tension and adjust it if necessary. Make sure it is not too high.
2. Upper thread is tangled: Check the upper thread for tangles or knots. Remove any tangles and re-thread the machine.
3. Upper thread sensor is dirty: Clean the upper thread sensor. Use a soft brush to remove any lint or debris.
4. Upper thread sensor is damaged: If the sensor is damaged, it will need to be replaced. Contact a Brother authorized service center for repair.
5. Incorrect upper thread path: Check that the upper thread is correctly threaded through the machine according to the user manual.
6. Defective upper thread sensor: If the above steps do not resolve the error, the upper thread sensor may be defective and needs to be replaced. Again, contact a Brother authorized service center for repair.