MAC air 2012 Office 2021

iOS is the operating system on iPhones.

iPadOS is the operating system on iPads.

MacOS is the operating system on Macs.

A mid 2012 MacBook Air can be upgraded to macOS Catalina 10.15.7 but no later. See the following from Identify your MacBook Air model - Apple Support

MacBook Air (13-inch, Mid 2012)

Model Identifier: MacBookAir5,2

Part Numbers: MD231xx/A, MD232xx/A

Newest compatible operating system: macOS Catalina

MacBook Air (11-inch, Mid 2012)

Model Identifier: MacBookAir5,1

Part Numbers: MD223xx/A, MD224xx/A

Newest compatible operating system: macOS Catalina

You may want to check the Microsoft forum for Macs for ideas about how to install Office on an older operating system such as Catalina. See the following: https://answers.microsoft.com/en-us/msoffice/forum/msoffice_word-msoffice_OfH-mso_OPMac?sort=LastReplyDate&dir=Desc&tab=All&status=all&mod=&modAge=&advFil=&postedAfter=&postedBefore=&threadType=all&isFilterExpanded=true&page=1

Microsoft has just bumped the minimum operating system for their Microsoft 365 applications in the Mac App Store to macOS Monterey (12.*). I expect that Office 2021 for Mac will now have an issue on older versions of macOS as they are now unsupported by Microsoft.