Invalid conversion when making a function that passes parameters to digitalWrite

Projects Programming
13 
  if(strcmp(relay, "relay1") == 0){
    if(strcmp(state, "LOW") == 0){
        digitalWrite(relay1, LOW);

That will work. You'd normally use some sort of table:

cmdType NameToRelayNo[] = {
  {"relay1", relay1},
  {"relay2", relay2},
  {"pump", relay2},  // another name for relay2
  {"relay3", relay3},
  {"lights", relay4} ...
}

If you really only want to accept names like "relayNN", you can convert the NN part to a number directly.

relayNo = atoi(&relay[5]);  // parse number starting at 6th character.