Go to System Preferences -> Security & Privacy
Select Privacy tab
Open the lock (enter your pwd when asked)
From the left side select 'Full disk access'
From the right side press plus button and add LibElec USB-CD Creator into the list on the right.
Quit LibElec USB-CD creator if open
Re-open LibElec USB-CD creator, select downloaded image or download one, etc...
Now writing to the USB/SD should work fine.
