The first place to check if you can't find your barrel nut is the back of the handguard. It is usually pretty hard to see and easily blends in. Loosen the 2 allen bolts and tilt your handguard so the muzzle end is upwards. The barrel nut should slide out.
If for some reason you still can't find your barrel nut after following the above steps, click the "Leave us a message" button and we will get you squared away. You can also email support at email@example.com.