I just spent the whole morning trying to figure this out. I don't understand it!!!
I currently have a LibreELEC (kodi OS) and it lists the VFS add ons. If I install kodi in Ubuntu via the kodi repo, I also can apt-get kodi-vfs-fstp and then it is listed in the add ons. This also works via an Ubuntu based Raspberry pie setup.
All that said, recalbox has been the best for actuall retro gaming, but I need to be able to add both sftp and a web browser to kodi.
As I said, this works fine via LibreELEC, but kodi gaming sucks. I have been unable to load the add ons. I found the zip file downloads, but they ALL gave the same error. Somthing about the wrong layout or somthing.
I had the same problem trying to connect kodi to my NAS. I managed to workaround this issue by defining the minimum SMB version to 1 and the maximum to 3 in the kodi SMBclient configurations. The minimum was selected to 'none' - then suddenly I could browse my SMB shares like I did in the previous recalbox version. I hope this helps!