I concur with Pureh@te. If "iwconfig wlan0 mode monitor" works, what is the problem?
If you don't understand why we are asking this, I think you have a fundamental misunderstanding of what "airmon-ng start" is supposed to do. You might want to look into the basics of how wireless sniffing/cracking works so you understand why it is you are trying to run this command. (Here is a hint: check the man page for airmon-ng, then then check what the iwconfig command listed above does.....)