Connect 2017 iMac to new iMac as 2nd display

The new iMac does not support using the old iMac as a display in Target Display Mode.

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

Plus the 2017 iMac can not be used unless it is running macOS Catalina or earlier.

You will need an external display if want a second display for the new iMac.

see > Connect an external display to your iMac - Apple Support

The 2017 iMac doesn’t support being a Target Display - period. Target Display Mode went away when the first 27” 5K Retina iMac came out in Late 2014, and never returned.

If it is running Catalina or earlier, the 2017 iMac can use another, even older, iMac as a Target Display. But that won’t be of much comfort to the OP.

Sorry. You can’t do that.