Command F2 doesn’t work

I am trying to use my mid 2011 iMac (21.5-inch) as a secondary display for my MacBook Pro.

I have read the following article, and connected the thunderbolt 2 cable to the iMac, however when typing command-F2 I am getting an error sound and nothing happens.

https://support.apple.com/en-us/HT204592

The macOS version I am using on the iMac is 10.13.6 (High Sierra).

I have tried Command-F2 both logged in and logged out of my session.

Would you know what is causing ? Is the command compatible with the version of macOS above?

Thank you!