So after I created my TGA file, I followed all the steps to try and convert it, and I get this error:
When I press a key, the window disappears, and I cannot find my converted VTF file anywhere. This is the file path I've been using for my TGA:
C:\Program Files (x86)\Steam\steamapps\common\sourcesdk_content\tf\materialsrc\signs
It's slightly different from the one provided in the OP, is that a problem?
The file path where I've been looking for the converted VTF:
C:\Program Files (x86)\Steam\steamapps\common\Team Fortress 2\tf\materials