I'm using Directory Opus 12.12.
When I select a webm file it plays in the viewer pane, but there's no image only a black screen. This happens only in the monitor I've connected to my Acer Nitro Laptop(which has full HD 1080 screen). When I move the directory opus to my laptop screen, the video plays just fine
That's an issue with the video codec in use rather than Opus itself. You could try switching to other video plugins to see if they use different codecs that work on both monitors, or updating codec DLLs. (The webm code is not part of Opus at all; it's a DLL something else has installed on your system.)
(Please also update to the latest Opus version -- 12.12 is quite old now -- and link your account.)