According to this, < your chipset does not supports monitor mode.
Not all combinations of hardware/software support wifi monitor mode. Like any other functionality implemented in silicon/firmware, be it listening to the media, sending and listening to packets, monitor mode has to be usually implemented by the manufacturer for it to work.
Beware that I far as I remember, some implementations only allow passive monitoring, while others allow monitoring _and_ sending/manufacturing "fake" packets.
Think it as monitor mode as a special/yet another service supported by the (firmware running on) hardware. When shopping around, that thought has to be taken in account, specifically for so more esoteric uses. So it it advisable beforehand to use Google, and talk with other people, to have an idea of what they are _already_ using successfully for specific purposes.
I would also direct you to another thread where I talk about my (bad) experience with cheap realtek devices.