I met the same error. I solved it. I first run the command:roslaunch MYROBOT_gazebo MYROBOT.launch, then in another terminal I run the command:rosrun gazebo_ros spawn_model -file rospack find baxter_description/urdf/baxter.urdf -urdf -z 1 -model baxter, then I can see the baxter in the gazebo world. I don't know the reason, I guess it's caused by the mechanism of spawn_model. Wish it help.