OK, here some  of the common problems I've had with MacOS:

1. MacOS Sierra seems to have developed this, when copying (but not moving) files on  a Synology NAS, I get the dreaded Can't move files, the required files cannot be found. Error code -43
2. When you try to delete in the Trash, you get the same error.
3. When you are on your network and connect to a Network drive, it gives a drive not found message.
4. The file is in use when you are trying to copy on a network drive.

The problem to each of these appears to be to kill Option Right click on Finder and choose Relaunch or kill it from the Activity Monitor with a force quit or logout and back and it should work again.