How can i get rid of the error "cannot convert 'String' to 'const char*'"

I want to get this code to recognise different commands and be able to parse the rest of the command, fore example I send "/sleep 5", and it will sleep for 5 seconds. However, I keep on getting this error "cannot convert 'String' to 'const char*'". here is my code:

#define maxCommandLength 64

String commands[] = {"/test", "/say", "/reset", "/sleep"};

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  Serial.println("Power on.");
}
char serial[maxCommandLength];
boolean valid;
void loop() {
  // put your main code here, to run repeatedly:
  Serial.println("Clearing command buffer(char 'serial'");
  for (byte i = 0; i < maxCommandLength; i++) {//if maxCommandLength is larger that 255, change 'byte i' to 'int i'
    serial[i] = ' ';
  }
  if (Serial.available()) {
    Serial.readBytesUntil(' ', serial, maxCommandLength);
  }
  if (serial[0] == '/') { //if it is a command
    Serial.print("Command recieved. \nCommand class:");
    Serial.println(serial);
    //test if command is valid
    for (byte i = 0; i < maxCommandLength; i++) {//if maxCommandLength is larger that 255, change 'byte i' to 'int i'
      valid = strcmp(serial,commands[i]);
      if(valid){
        break;
      }
    }
    //execution of command
    Serial.readBytesUntil(' ', serial, maxCommandLength);
    //.......
  }
}