I am running Kubuntu 18.04 with Plasma version 5.12.7, and the keyboard shortcut for terminal is switched off by default.
I didn't use KDE menu editor solution as I got a conflict warning message when I tried to edit the 'Current shortcut key'.
It said "The shortcut 'Ctrl+Alt+T' conflicts with the following combination: Shortcut 'Ctrl+Alt+T' in Application KDE Daemon for action Run Konsole"
Made me think there's already a setting somewhere that is switched off, which I eventually found going through System Settings. (click launcher button, start typing system (for System Settings) click the link when it appears) to launch System Settings.
In the left panel menu, open 'Shortcuts', under the Workspace category,
open 'Custom Shortcuts'
in the next panel,
activate 'Example' button (it's greyed out by default - see the comments tab once 'Example' is selected)
Now you can click 'Example' to open sub menu drop down,
Activate "Run Konsole" button
Click apply and close out of the screen.
Now Ctrl+Alt+T works.
Shutdown and restarted just in case - Still works.