Black bars on external display
The vertical black bars appear when the MacBook screen resolution and the external display screen resolution is not in the same ratio. 

To fix that, when you connect to the screen, there is a prompt to choose which screen resolution you want to use. It’s either MacBook or the external display. This is how vanilla MacOS works. 

Or, go Apple Menu --> System Preferences -->  "Displays" pane  set your preferred monitor resolutions there.