Did you install Steam (and the Steam Library with the game) into a Windows "Program Files" folder? Windows "security" will stymy any attempts to alter files in that folder with mods. Refer to
Moving a Steam Installation and Games from the Steam Support site for ways to do that. If SH5 is the only game, then it is probably easier to unintall the game and Steam and start over. If you have a 2nd, valid HD that you can put a Steam Library on, then you could do that much easier, and just re-install the game there. If you have to re-install Steam because of only having the C: drive available to use, then you would edit the install path during the install process, and direct it to install into a folder of your own making, such as C:\Games, which then would give you a path of "C:\Games \Steam..." or whatever you choose. Most of Steam itself still ends up in the Program Files folder, but your Steam Library will be in that "C:\Games..." structure. You might have to also download and update UConnect from Ubisoft yourself. Be sure and follow vdr1981's TWoS Steam instructions, and then you should be able to get through the GFP program all of the way. Then it will just be a matter of figuring-out to efficiently get down the ladder in the sub so that you can dive the boat when necessary...
lol