Firstly the modern car radio is generally integrated into the vehicle security system and when powered up must recognise the vehicle VIN number programmed into the vehicle security system. Lots of such radios no longer use a security code as it is deemed no longer necessary.

Where a 3 or 4 digit security code is used the radio records the number of attempts to enter the code and each subsequent attempt causes an increasingly long pause before another attempt can be made so that the ninth attempt can mean leaving the radio switched on for several hours before the dashes reappear and then if the tenth attempt fails the radio is permanently locked and effectively scrap as dealer expertise is expensive.
Not only must the correct code be entered, it must be entered in the correct way so it is always best to refer to the vehicle audio manual.

The only time I have ever seen ERROR on a radio it was an example of a permanently locked unit...
I might be wrong so again it is best to refer to the audio manual.

Try engine running. Switch off radio. Press and hold 2&5, then switch on radio