Command F2 Fn function not working

There is no way to use your iMac as a first-class hardware monitor for your MacBook Pro.

There are “workaround” products, which may involve compromises. One is Luna Display (by AstroPad). Another is Duet Display (“subscription”, a.k.a. rental, software).

Target Display Mode went away when the 27” Late 2014 5K Retina iMac - your iMac - came out.

Even if Apple had not later added restrictions on the video source, your iMac could not be used as a Target Display.

calebfromsnelling wrote:

Appreciate you! Any way to get a 5k monitor display without breaking the bank?

I only know of three standalone 27" 5K (5120x2880 pixel) monitors.

• Apple 27" 5K Studio Display. High-quality, similar to the display of your 27" 5K iMac, but starts at $1600. By the time you add an Apple Magic Mouse and Magic Keyboard, you're into the old price range of 27" 5K Retina iMacs … except that you still need to buy the computer!
• LG 27" 5K UHD UltraFine™ IPS Monitor with macOS Compatibility. It lists for $1300, but recently, I saw a place selling it for $850.
• Samsung 27" ViewFinity S9 5K IPS Smart Monitor. It lists for $1600, but I saw a place selling it for $800.

Mac's after 2019 including your new MacBook Pro, do not support using any iMac in Target Display Mode.

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

You're welcome..

Best bet is to use an external display with the MacBook Pro.

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