How to remove a Volume using Terminal?
What looks like a folder in /Volumes is not a real folder.  Everything inside /Volumes looks (at first glance) to be folders, but in reality are what are called mount points.  These are just places to join disks into the tree of folders starting from /Volumes.
When you mount a disk, macOS creates a mount point in /Volumes and connects the disk to that mount point.
When you cleanly dismount a disk the mount point is removed.
But sometimes the mount point gets left behind.  For example, following a physical disconnect without a preceding dismount.
The last few comments in this blog post show someone who has 3 such mount points left behind. Howard Oakley's blog
You have two choices:

Don't worry about it.  The mount point is not taking up space on your disk.

Attempt to remove the mount point.


Howard Oakley advises not to mess with it. The danger when tampering with /Volumes is that you will mess up your whole folder structure and end up with an unbootable system.