Kismet stops capturing or returns "invalid command" after a while
dr
It looks like several random problems might be tied to one root
external cause.

If Kismet stops capturing packets a little while in, or after running
for a minute suddenly says it cannot set the channel, make sure you're
not running any wifi management software like NetworkManager.  It appears
this resets the card mode when it looks for networks to join.

If your drivers return an error on setting channel in management mode
(which is pretty normal), it will cause Kismet to fail immediately.
If they don't return an error and silently ignore channel setting, then
it will keep running but never see an 802.11 packet so no more
packets will show up.