Xcode can’t be installed on “Macintosh HD” because OS X version 10.14.4 or later is required.

Before you try to upgrade to Catalina, it's important to know exactly which MacBook Air you have.

Click the Apple menu > About This Mac and have a look at the panel that presents. It will tell you which year Mac you have.

Then check for compatibility with macOS Catalina here:

macOS Catalina is compatible with these computers - Apple Support

edit - disregard the suggestion above here re: Catalina. I just realized you're wanting to get to Mojave 10.14.4.

Must get my mind back in the game. 😃

Back up your data, check your applications for compatibility, and download Catalina.

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There is only one MacBook Air model which matches those specifications and will be on 10.9.5; it's a mid-2012 one.

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