Just guessing here, but...

Usually when I see this type of error, I have to ask if you have downloaded and installed the latest Blender Source Tools add-on. Most times when error like this occur, it's because someone is trying to use BleST 2.10.2 in a version of Blender greater than 2.79b (usually caused by carrying forward preferences from 2.79b into 2.80 or later when the new version is first installed or by installing old versions of BleST from downloads stored somewhere on system drives.)

Current version of BleST for 2.80 and above is still 3.1.0 available here. https://steamreview.org/BlenderSourceTools/

or if you want the latest unstable (being work on) version you can grab the master from the GitHub here (using the Release link here you can find the last stable 3.1.0 as well)

https://github.com/Artfunkel/BlenderSourceTools

Make sure you have the proper version loaded for the version of Blender you're using.

If you are trying to export using 2.83, then why is Blender Source tools in a folder for 2.91

"C:\Users\*****\AppData\Roaming\Blender Foundation\Blender\2.91\scripts\addons\io_scene_valvesource\export_smd.py", line 207, in execute

The VTA vertices object is loaded with vertex information of vertexes that didn't match flex vertexes and is not required, you can delete that object or just remove it from your QC.

You have mesh in the model that has not been assigned to a material or the material has not been assigned to a texture. (this is normal coming out of an older version because of the material system rewrite in the newer version and the materials not using nodes),

Select the 4 objects, go into edit mode - face select mode then start selecting mesh from the materials tabs. When you've finished selecting the mesh, the left over faces are the ones you need to assign to a material. Once they are assigned, re export and recompile.

Or make sure you have the proper version of BleST for Blender 2.66 installed, if you have a version newer than 2.1.0, you're going to get errors like this.

Or as Zeq suggested, you can load a blend file created in Blender 2.66 into the newest version of Blender with little to no consequences just by opening the blend file in the newer version.

When you point BleST at the folder your QC is in it should create a button with the name of the QC on it. Use that button, not the Compile All button.

It all depends on what you want to export.

I sent you a FR, best to take this offline now, especially out of the BleST group discussions. BleST still has nothing to do with the problem.

Just edited my last response.

Actually, the PNG texture ARE here. After doing an exhaustive search for them I found them in my Steam\steamapps\common\Blender\textures (where steam installs Blender) folder. I had tried to do an unpack as well and could not find them. SO, this IS a valuable add-on right out of the box.

Save your Blend file then cut and paste that textures folder into the same folder you saved the blend file. Close and reopen the blend file

No, it's an fantastic add-on, I might send the author a note to see if he can fix this or if I'm missing something in my evaluation. Thanks for bringing it to my attention.