Please upload the whole scene and I'll see what's going on. It's unlikely that anything can be done in the export script to avoid this, but I might be able to improve the error message to give a better suggestion of the cause.

Ahh, "GarrysMod" just isn't recognised as an engine release. I'll need to add it. In the meantime you will have to manually select a DMX format and version. Please let me know what the correct values are, if you find them.

You can import as many animations as you want, and they are all stored in the scene as Actions.

Unfortunately Blender's UI makes it hard to manage multiple Actions, so I added a section to the Source Engine Exportables panel that makes switching easy. Select the armature to see it.

Do you have the latest version of the tools? I made a preview release at one point which had a bug where the engine path was never validated.

You need to select a folder containing studiomdl.exe.

2 second web search: https://docs.blender.org/manual/en/latest/editors/nla/index.html

The tools export the full scene without regard to hidden objects or frame subsections, and this is by design.

You should be able to achieve this by setting up Non-Linear Animation (NLA) in your scene. This way you can select a subsection of the action either by cropping it, or by moving its track segment before scene frame 0.

Whoops, fixed this and released 3.2.2.

It looks a lot like the mesh & bones were imported with one up axis and the animation with another.

This is intentional. Units in SMD and DMX do not relate to any real-world measurement. Both the importer and exporter write whatever values they find in the scene at 1:1.