The action can't be completed because the folder or a file in it is open in another program. Close the folder or file and try again. This is done for good reason.

As the folder creator, you can control who can view or edit anything in a shared file or folder. Here’s how to share a Dropbox file or folder. Read more: What is Dropbox, and how do you use it?

For file and folder management, that number dwindles. I've already listed what I believe are the 5 Linux commands every user should learn, all of which are also related to file and folder management.

Think of the top-level folder as a drawer in a file cabinet. You might have one drawer for work, one for household, one for side projects.