What is the latest OS version I can install on a 2015 MacBook Air?

Monterey.

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Click here and follow the instructions. If you find that the installed RAM(not free disk space) isn't enough to effectively use it, you'll need to downgrade or get a computer with more.

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I tried Montery and it said I was 6GB short. i have a MacBook Air 7,2. 13" 2015 model, memory 8GB.