The E21 error on a Brother embroidery machine indicates that the machine is experiencing a problem with its lower thread 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 lower thread. Make sure the thread is properly threaded through the machine and that there are no tangles or knots.
3. Check the bobbin. Make sure the bobbin is properly inserted and that it is the correct type and size for the machine.
4. Check the lower thread sensor. Make sure it is clean and free from any debris or dust. If it appears to be damaged, it may need to be replaced.
5. Check the embroidery design. If you are using a pre-loaded design, make sure it is compatible with your machine. If you have created your own design, check it for any errors or issues.

If these steps do not solve the E21 error, it may indicate a more serious problem with the lower thread sensor or the machine’s internal components. In this case, it is best to contact a qualified technician or the manufacturer for assistance.