Rename Mac Directory in Terminal

Just to be clear here. Is this a folder within your home user folder or is it the short user name for your macOS user account for the home folder?

If you need to change the name of your macOS user account and/or home user folder, then use the directions in this Apple article (make sure to perform each step carefully or you will lose access to your data at least temporarily -- perhaps even permanently depending on whether the Mac requires authentication for Recovery Mode):

Change the name of your macOS user account and home folder - Apple Support

When using the command line to rename an item you actually must use the "mv" (aka "move" command), but before I give you the command you need to clarify whether this folder is within your home user folder or the home user folder itself.

Also, I highly recommend you have a good backup before doing anything just in case something goes wrong.

You are likely not in the correct directory, so it can't find "oldname." You would need to

cd /full/path/to/parent/folder

You have to replace "/full/path/to/parent/folder" with the actual folder path.

If you can't rename it in the Finder, you won't be able to rename it in Terminal.

Which directory are you trying to change? Where is it located.