This is an indicator of macOS not having cleaned up a previous mount point. This is usually solved by rebooting, but can also easily be fixed manually:
Mount point /Volumes/Drive - [team name] already exists.Please unmount or remove it.
Go to /Volumes
There's an empty Drive - folder, delete it (password required)
Go back to Postlab, and connect Drive
Note: if this issue returns on the same computer, there's most likely a permissions issue. Get in touch.