The charger has detected that the Live and Neutral conductors are swapped in the supply wiring.
Common causes
Installation wiring error — L and N connected to wrong terminals
What to check
STEP 1
Power off at MCB. Check wiring at charger terminal block — L=Brown/Black, N=Blue, PE=Green/Yellow for single-phase UK supply. Correct polarity and restore
STEP 2
Verify polarity at consumer unit before reconnecting