Can I use Imac as monitor for mac book pro

No, only the late 2009 and 2010 27" iMacs supported Target Display Mode (TDM), if you need an external display for your MBP, then you need to buy one. As your question is common, we have done substantial research on this and there are no work arounds for TDM.

Read this reply by Allan Jones in another topic about Target Disk Mode:

Allan Jones

User level: Level 9

(50,524 points)

Nov 9, 2020 3:08 PM in response to douglitas

SOURCE computer (The one sending out the video signal to an external): 2019 or older. Can run up to macOS Catalina 10.15

TARGET computer (the one to become the external monitor): Can run no higher OS than 10.13 and only within this narrow model range:

Late 2009 to Mid 2010: Must be 27 -inch

Mid 2011 through Mid 2014: can be 21.5 or 27 inch

Apple started the deprecation of Target Disk Mode six years ago and finished it off in early 2020. It's dead.

It is doubtful that a new macOS version will retain ANY support for TDM regardless of computer model. Even the support article is archived and will not be getting any more updates/