Welcome to the forum!
The reason the path operation won't work, is because the path is technically a Group of 3 paths. (You can see that by selecting the path with Selection tool, and looking at the status bar.) Once you "get around" the group, you can use path operations.
To "get around" the group, you can either Ungroup, or do something called "entering the group". Double-clicking on it will enter the group. I usually don't recommend entering the group for newbies, because it can get confusing. But if you get stuck, just let us know.
I would suggest using Path menu > Cut Path, rather than the others. That will break the path at the place where the square intersects with the path. Then you can just delete that piece. (Using other path operations, Inkscape will try to close the path, which will give you an extra segment which you then have to get rid of as well. That's because the path operations generally work on the area inside paths, rather than the paths themselves....generally, but not with Cut Path.)
