Had a similar problem. As it turned out for some reason the Intel graphics driver's runtime component had locked the directory. This never happened on another Windows 10 system that didn't have that driver on it for me. If you go download Process Explorer from MS and then use the find function, search for 'kerbal' and see if there is a process locking it. After I killed the process tree locking it I was able to move\delete folders so I ended up just moving my entire Steam folder to the root of the drive instead of having to deal with this again.
Denken
