Connecting iMac to other iMac for target display mode

No can do, my friend. The Mac that you want to connect to the target iMac must have been released in 2019 or earlier and running Catalina or earlier.

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

Target display mode is a feature available only with certain older iMac models.

Only the following older iMac models can be used as an external display when plugged into another older Mac model:

• iMac models introduced in 2011, 2012, 2013, and mid 2014 using Thunderbolt or Thunderbolt 2 cable.
• 24-inch and 27-inch iMac models introduced in 2009 and 2010 using Mini DisplayPort cable.
• The intended target iMac must be running macOS 10.13 High Sierra or earlier; it does not work with macOS 10.14 Mojave or later, or with Boot Camp and Windows.*
• Retina display iMacs won’t work as target displays.

• In all cases the other Mac that you're connecting it to must have been introduced in 2019 or earlier and have macOS 10.15 Catalina or earlier installed.

Hi Stubojug,

Welcome to Apple Support Communities.

Sorry. You cannot use Target Display Mode. The output Mac needs to be 2019 or earlier, running macOS Catalina or earlier. Yours is a 2021 model.

-Jack

A SMART error means your drive is about to fail.

And even if you could do the downgrade, you can't change the fact that the source Mac is 2021, and not 2019 or earlier, and that won't work.

-Jack