iMac 27 - late 2019

From the link I posted…

24-inch and 27-inch iMac models introduced in 2009 and 2010

To use any of these iMac models as an external display:

• The iMac used as a display must have macOS High Sierra or earlier installed.
• The other Mac that you're connecting it to must have been introduced
• in 2019 or earlier and have macOS Catalina or earlier installed.
• The cable connecting the two Mac computers must be a Mini DisplayPort  cable.

No

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

What year is your iMac? "iMac 27 - late 2019" indicates it's from 2019.

You're welcome.