I've tried all the instructions for using Target Display Mode with an older iMac being the display and a Macbook Pro as the driver. all the right OS versions. still can't get the F2 function key to work, even w Fn

Welcome to the Apple Support Communities.

Using this Apple Support article as a guide, About System Information on your Mac - Apple Support, please let us know, for both Macs, what year they are and what version of macOS they are using.

Jack

A couple of things, you are not specific about the model years and versions of Mac OS installed on either machine. Next, did you know Target Display Mode has been dead for about 8 years (2014), please refer to Target Display Mode (TDM)

Hi there, I have a Macbook pro 2020, and an imac 2012 - I have bought the adapters but F12 function is not working. Is this because my Imac is updated to catalina?

what do I do know? I have just spend $130 on useless cords...

From the article above your name:

Return the cables, Target Display Mode will not work. Use the return l money to buy Ann external display.