how to blur background on my Mac Book Air version 11.3

If you want to have a blur background on your Mac, you must meet the system requirements for the virtual background (Image without a green screen) 

• Zoom desktop client, 4.6.0 or higher
• macOS version 10.13 or higher
• Supported processors
  • Apple M1  
  • i5 - 2 cores or higher, 6th generation or higher
    Note: 6th generation i5 dual-core processor, requires macOS 10.14 or higher
  • i7 - 4 cores or higher, 4th generation or higher

You may also refer to this article: Zoom Virtual Background system requirements 

Hope that helps and please make sure to mark the solution as accepted if this information is what you needed.