Imac 27 2011 as secondary display for Macbook Pro 14 2021

Hi vurdalak,

We’re here to lend a hand with your MacBook Pro and using the iMac as a secondary display.

Looking at the article you posted your question using, Use your iMac as a display with target display mode, it shows the requirements to use the iMac as a second display are: 

• 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 Thunderbolt or Thunderbolt 2 cable.

Let us know if you have further questions.

Take care!