There is nothing wrong with your command. Your command is waiting for an input and that's the only reason why it doesn't get executed!
For instance:
$ awk -v AVG=$(uptime|awk '{print $(NF-2)}') '{ if ( AVG >= 0 ) print "there is a load" }'
abc ## I typed.
there is a load ## Output.
Just include BEGIN in your command as suggested by the experts!
$ awk -v AVG=$(uptime|awk '{print $(NF-2)}') 'BEGIN{ if ( AVG >= 0 ) print "there is a load" }'
there is a load