The F9 error on a Brother embroidery machine indicates a problem with the lower thread tension sensor. Here are some steps you can take to fix the issue:

1. Turn off the machine and unplug it from the power source. Wait for a few minutes before plugging it back in and turning it on again. This can reset the machine and clear any temporary glitches.
2. Check the bobbin. Make sure the bobbin is properly wound and inserted into the bobbin case. Make sure the thread is properly threaded through the bobbin case and there are no obstructions or tangles in the thread path.
3. Check the lower thread tension. Make sure the thread tension is set correctly for the type of fabric and thread you are using.
4. Check the lower thread tension sensor. Make sure the sensor is clean and free from any debris or dust. If it appears to be damaged, it may need to be replaced.
5. Check the machine for any signs of damage or wear. If any parts appear to be damaged or worn, they may need to be replaced.

If these steps do not solve the F9 error, it may indicate a more serious problem with the lower thread tension sensor or the machine’s internal components. In this case, it is best to contact a qualified technician or the manufacturer for assistance.