iMac system requirements for AirPlay

See this for the system requirements for airplay to Mac

AirPlay to Mac works best with these devices and operating systems.

• iOS 14 or latermacOS Monterey or lateriPhone 7 or later
• iPad Pro (2nd generation) or later 
• iPad (6th generation) or later
• iPad Air (3rd generation) or later
• iPad mini (5th generation) or later
• MacBook introduced in 2018 or later
• MacBook Pro introduced in 2018 or later
• MacBook Air introduced in 2018 or later
• Mac mini introduced in 2020 or later
• iMac introduced in 2019 or later
• iMac Pro
• Mac Pro introduced in 2019 or later
• Mac Studio introduced in 2022 or later

You can AirPlay to Mac at a lower video resolution from older iPhone, iPad, and Mac models and operating systems if the supported Mac has the AirPlay Receiver setting in Sharing settings configured to allow AirPlay for either Everyone or “Anyone on the same network”.

Are you asking about using AirPlay to stream from a Mac to an Apple TV box? Or using AirPlay to stream to a Mac?

The latter feature is newer, and so not supported by as many Macs and versions of macOS.

The answer is here:

Use AirPlay to stream audio - Apple Support