How can I activate AirPlay on my iMac, High Sierra 10.13.6, so as to stream to AirPlay 2 on Samsung 7?

AirPlay 2 system requirements

The following devices support streaming audio with AirPlay 2. If your device meets the requirements below, but you can't use AirPlay, learn what to do.

Devices you can AirPlay 2 audio from

• iPhone, iPad, or iPod touch with iOS 11.4 or later
• Apple TV 4K or Apple TV HD with tvOS 11.4 or later2
• HomePod with iOS 11.4 or later
• Mac or PC with iTunes 12.8 or later 
• Mac with macOS Catalina

Devices you can AirPlay 2 video from

• iPhone, iPad, or iPod touch with iOS 12.3 or later
• Mac with macOS Mojave 10.14.5 or later

Devices you can stream to

• Apple TV 4K or Apple TV HD with tvOS 11.4 or later
• HomePod with iOS 11.4 or later
• Speakers connected to the Audio Out port on AirPort Express 802.11n (2nd Generation) with the latest firmware update
• AirPlay 2-compatible smart TVs1 with the label:
• 
• 
  
• Speakers and receivers with "Works with Apple AirPlay" on the speaker packaging3
• 

https://support.apple.com/en-us/HT208728

https://www.crutchfield.com/S-Lyu2UTJZYAn/learn/what-works-with-airplay-2.html

OK, great news I could help, take care my friend. :)