Hi，I have the same problem, I can't see my device, let alone delete it, and arduino create reminds me that I have opened 3 devices, I can't add devices anymore.
pert:
It should be Arduino Manager for Linux.Do you mean the page that has "Add new board", "Add new device", "Add new SIM"?:
If you're on that page and you don't see any devices listed, it just means you don't have any devices and you need to add some.
