It means that another software is using the tun/tap interface.
Please do the following:
Enter "keststat" in your terminal.
It will give you a list of all tun/tap interfaces( they contain the "tun" in their name)
After that you should deactivate this interfaces. For example:
For kext com.viscosityvpn.Viscosity.tun you should do:
"sudo kextunload -b com.viscosityvpn.Viscosity.tun" in your terminal.