Add-on was properly installed each time, and I gave blender a restart after installing, before trying to export, each time.

Please upload your blend file.

You need to create a keyframe animation for your armature. Press ctrl+tab with your armature selected and press i > Location, Rotation and scale. Hope it helps, it worked for me

^ Then export everything (you can later delete this animation). I recommend to use proportion trick for any of your custom humanoid armatures if they are about using playermodel native anims

One keyframe is enough

Originally posted by Artfunkel:

Please upload your blend file.

ErlGrey's answer worked for me but here's the blend if you want to search for other problems.
I was attempting to only export the head mesh and armature.
https://www100.zippyshare.com/v/AsAIv7Lv/file.html

Originally posted by Artfunkel:

Please upload your blend file.

**UNMARKED AS SOLVED**
ErlGrey's answer allows the .dmx to export but once imported to Source 2 modeldoc, it is stuck facing one direction no matter how it's rotated before export. It seems like a sorta working hacky fix but isn't fully viable at least in my attempt

Works for source one (Gmod atleast). Have you tried modifing $origin in qc file?

I can't download your file and can't reproduce the issue locally, so I don't really know what caused this. But I was able to put some checks into the code to handle things being unexpectedly null.

Thank you! Not sure why you can't download the file, maybe zippyshare limitation? I'll give this export another shot

The changes haven't left my computer yet. Please try to upload your file to another site so that I can test whether I've actually fixed the issue.

https://we.tl/t-n34g5jIfjx should work

Python: Traceback (most recent call last):
File "C:\Users\Марк\AppData\Roaming\Blender Foundation\Blender\4.1\scripts\addons\io_scene_valvesource\export_smd.py", line 209, in execute
self.exportId(context, id)
File "C:\Users\Марк\AppData\Roaming\Blender Foundation\Blender\4.1\scripts\addons\io_scene_valvesource\export_smd.py", line 551, in exportId
self.files_exported += write_func(id, bake_results, self.sanitiseFilename(export_name), path)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Марк\AppData\Roaming\Blender Foundation\Blender\4.1\scripts\addons\io_scene_valvesource\export_smd.py", line 1028, in writeSMD
anim_len = animationLength(ad) + 1 # frame 0 is a frame too...
^^^^^^^^^^^^^^^^^^^
File "C:\Users\Марк\AppData\Roaming\Blender Foundation\Blender\4.1\scripts\addons\io_scene_valvesource\utils.py", line 311, in animationLength
if ad.action:
^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'action'

wtf

Blender 4.1 has broken sharp edges, so use 4.0 or below.

Originally posted by ErlGrey:

Blender 4.1 has broken sharp edges, so use 4.0 or below.

not technically broken, just moved. the auto smoothing has been replaced with a modifier called "smooth by angle" or something.