How to set the Virtual Background in lib-jitsi-meet

there is no option to set the virtual background in lib-jitsi-meet library

That is a feature implemented at the app level. It’s integrated with LJM using the setEffect API of JitsiLocalTrack.

whwre to get the effect instance to pass in setEffect function ?

You need to supply that yourself. Our implementation is here: jitsi-meet/JitsiStreamBackgroundEffect.js at master · jitsi/jitsi-meet · GitHub