The tag tagname in containername must have a texture file extension.
For example, the config file text
kind "industry" trainz-build 3.5 light 1 nightmode "lamp" icon-texture "industry_icon.tga"
would be flagged with the error
! VE176: The tag 'icon-texture' in 'industry' must have a texture file extension.
The fix is to create a file 'industry_icon.texture.txt' with file contents
and change the line in config.txt to