ERROR: cannot launch node of type [mavros/mavros_node]

I am trying to use gazebo to make a virtual quadrotor fly. I enter this command: roslaunch mavros px4.launch fcu_url:=“udp://:14540@127.0.0.1:14557”

and I receive this error: ERROR: cannot launch node of type [mavros/mavros_node]: can’t locate node [mavros_node] in package [mavros].

Please help! Thank you.