Generic question about the error "Waiting for service /gazebo/spawn_urdf_model"

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I wouldn't necessarily call it an error, but rather a information message. Your launch file is trying to use gazebo's spawn_urdf_model service and it is either currently unavailable (e.g. gazebo server has not yet been started) or the service itself is hanging for whatever reason. In my experience, the latter is the most common reason and usually boils down to one of two common causes.

• files referenced within your urdf file cannot be located on Gazebo's paths. When Gazebo can't find a file or a model, it looks online to the gazebo model database. This can be a slow process and can give the impression that Gazebo is frozen for some time. It should eventually fix itself and you should be able to notice missing models or textures on whatever does spawn. If you run gazebo in --verbose mode, you may see some output indicating this is the case. You may also be able to find information on this in the ~/.gazebo/ogre.log or the ~/.gazebo/gzserver/default.log
• alternately, your computer just doesn't have enough power. This has been common enough for me when running gazebo in a virtual machine, that didn't have enough graphics memory and RAM allocated. The fact that the terminal itself freezes maybe points to this, but its hard to say

Its been a while since I've used melodic, so there may be other issues that I don't remember. But the above may offer a place to start looking.