Looks like you are trying not to install in the official sourcemods folder. Tell me the exact path you entered for installation.
Here my folder, I think it's the correct one: "C:\Program Files (x86)\Steam\SteamApps\sourcemods"
Just to let you know, my steam folder is installed on another SSD and on another partition...
Anyway, I've finally succeeded in installing the mod and using it by unpacking manually the "BigFile.7z"... and strangely it works for me now
thanks to all of you
My new problem is purely collateral and minor, I'm now unable to play TF2 because of an engine error I guess (see my captures if you have time