When you refill toner while the printer is powered off, the printer may not detect the amount of toner and the error message can’t be cleared.

If the error can’t be cleared, turn on the printer, and follow the toner refilling procedure with an empty cartridge. The printer detects the amount of toner and the error message will be cleared.
In case it doesn’t fix the problem, the amount of toner may be low. Refill the toner.