iMac as an external display

Target Display Mode only worked with certain older year model Mac's.

carefully read > Use your iMac as a display with target display mode - Apple Support

For ease of use and reliability, your best bet is to just get an aftermarket display for your MacBook Pro.

see > Use an external display with your MacBook Pro - Apple Support

If the Target Display Mode article does not provide clarity, please post the sub-models of both the iMac and the Macbook Pro, from "About this Mac.." and their respective OS versions. Examples:

There are very stringent requirements for both hardware and OS versions for the now-deprecated Target Display Mode to work.

If all else fails look into:

Luna Display | Turn your Mac or iPad into a second display (astropad.com)

and/or

Duet - Remote Desktop, Second Display, Drawing Tablet, and More (duetdisplay.com)

I've never used either on so can't comment on them further.