It looks like you have missed for the "rospack find MYROBOT_description". Things inside will be executed by bash first and output the result to replace the command. You should modify your command to this as the tutorial gives:
rosrun gazebo_ros spawn_model -file rospack find MYROBOT_description/urdf/MYROBOT.urdf -urdf -x 0 -y 0 -z 1 -model MYROBOT

And remember to source your project first so that bash can find out the package location. And I think you can try running $roscore and see if the "Unable to register with master node [http://localhost:11311]" error occurs again or not. Normally, roslaunch will run roscore automatically. It looks like you have missed  for the "rospack find MYROBOT_description". Things inside inside it will be executed by bash first and output the result to replace the command. You should modify your command to this as the tutorial gives: rosrun gazebo_ros spawn_model -file rospack find MYROBOT_description/urdf/MYROBOT.urdf -urdf -x 0 -y 0 -z 1 -model MYROBOT  And remember to source your project first so that bash can find out the package location. And I think you can try running$roscore and see if the "Unable to register with master node [http://localhost:11311]" error occurs again or not. Normally, roslaunch will run roscore automatically.