What exactly do you want to make? When you say “blur effect on pause menu”, I assume/guess you want to blur the background. If that is the case, there are some tutorials and threads on this forum where the topic is discussed.
Which rendering pipeline are you using? Built-in, URP or HDRP?
In any case, here are two links (there’s more of course) which you should be able to find yourself if you just type to google “Unity blurred menu background” or something similar.