MacBook Air (11-inch, Mid 2012)

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

Safari will open a redirect and open the Apple Store where the download can commence.

macOS Catalina 10.15 hardware requirements This might be the Last Version of macOS this computer Qualifies to run

macOS Mojave 10.14 hardware requirements

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS

Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina, Big Sur & Monterey supports only 64 bit and there is no workaround.

Suggest using Go64 to find 32 bit application before attempting the upGrade. Some or many of the Application already installed may required update or upGrades to 64 Bit Versions.

dualegas Said:

"MacBook Air (11-inch, Mid 2012): I have difficulty to download outlook and office 365 into my MacBook Air (11-inch, Mid 2012)"

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Incompatibility being the Culprit:

For a much older Mac, it seems you need a much older version of Microsoft Office. So, what is the macOS version on your Mac? Find out going to: Apple Menu > About This Mac > Top Item.