Not able to connect an external acer display to MacBook Pro 2015
If the external screen is flickering and blacking out after 3-5 sec, try going into System Preferences > Displays and change the settings for your external monitor. I solved mine by setting the Resolution to 1080p and Refresh rate to 60 Hertz.

P.S. You can bring the display settings window of the monitor to your main display on the Mac by hitting 'Gather Windows'.