Rather simple in fact:
In Content Manager ..."Setting", tab "Content Manager", choose General at the left and the first line mentions: Assetto Corsa Folder .... check if your path is similar to this:
.....\Steam\steamapps\common\assettocorsa ... and nothing more after this.
If it's not correct, click on "Change" and you'll get a message saying the app needs to be restarted.
NB: I didn't go further ... but you'll have to choose the new path at the same place after the restart.
That should be working afterwards ..... except if the problem is elsewhere.