Don't worry, this is a common question

Ok, as you know, Crobat is a GSC generation Pokemon, while Golbat is RBY. Your Pokedex is only Pokemon 001-151 at the moment, so in order for your Golbat to evolve, first you must obtain the National Dex, which you can only get after defeating the Elite4. Once that is done, and you get the National Dex, Golbat will evolve ^^