The E91 error on a Brother embroidery machine indicates a problem with the communication between the machine and the computer. 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 USB cable. Make sure it is securely connected to both the machine and the computer. If it appears to be damaged or frayed, it may need to be replaced.
3. Check the embroidery software. Make sure it is compatible with your machine and that it is set up correctly. Check for any updates or patches that may be available.
4. Check the computer’s settings. Make sure the computer is set up correctly to communicate with the embroidery machine. Check the device manager to see if the machine is recognized.
5. Try a different USB port. If the USB port you are using is not working correctly, try using a different one.
6. Try a different computer. If the issue persists, try connecting the machine to a different computer to see if the problem is with the machine or the computer.

If these steps do not solve the E91 error, it may indicate a more serious problem with the machine’s internal components or the computer’s USB port. In this case, it is best to contact a qualified technician or the manufacturer for assistance.