Why do I get "invalid operands of types 'const char*'" error? I am making an Arduino alarm. This is the error:

exit status 1
invalid operands of types 'const char*' and 'const char [2]' to binary 'operator+'


Here is the code that triggers it: 

lcd.print("0"+hours+":"+minutes);


This code is similar, but worked before:

String none="00";
lcd.print(none+":"+none+":"+minutes);


I can post the full code if you need it. How can this be fixed?
Your first code took a String object, then added other strings (note lowercase) to it - which are characters enclosed in quotes. It is legal to add strings to String objects.

Your new code doesn't have any String objects at all - and you're not allowed to add strings to strings (both lowercase).

It will work if you change your new code to:

lcd.print(String("0")+hours+":"+minutes);

That turns the original string "0" into a String object, and then adds more strings to it.