Connect my 2015 Macbook Pro to my 2019 iMac as an external monitor
Some newer iMac models cannot be used as external monitor for another Mac.
- Use your iMac as a display with Target Display Mode - System Requirements
iMac (Retina 5K, 27-inch, Late 2014) and later iMac models can't be used as Target Display Mode displays.
Nimesh Neema is correct, you cannot use your iMac in Target Display Mode as this feature has not been supported in iMacs manufactured for about the past five years.
However, there are some software options you can investigate, but I have no personal experience with them. Two of these options are listed below:
- Air Display
- Mirroring360
In summary, they promise to allow users to share their Mac screens on a variety of devices, including other Macs.
Apple has dropped Target Display Mode support to the 5K iMacs since the bandwidth restrictions when it debuts. There are news mentioning the Intel chip or whatsoever giving further restrictions to the later iMacs and Apple has officially removed TDM support since 10.13.
You may use a pricey workaround like game capturing device to display (and capture) HDMI signal as an app on iMac. It is initially for YouTubers’ use.