The “ERROR 21” message indicates the abnormality of motor current, which is normally caused by over-current. You can proceed as follows:
Step 1:
Locate the controller, perform an overall check for all the cables that go from the controller. The controller is on the top tube, near the folding position. Initially check if any cable sheaths are damaged or corroded.
Step 2:
To access the terminals of the controller, fold the bike and remove 4 screws to demount the contact panel of the controller carriage. Check if the cables are broken or burned.
For further assistance, please be sure about the cases listed above and reach the G-force support team at: support@g-forcebike.com