mojave over sierra

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.

macOS Mojave - Technical Specifications

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

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.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal  operation of the OS and may even Corrupt the OS requiring a Reinstallation

Back up your data, check your applications for compatibility, and if your MacBook Air is a 2012 or newer model, click on Mojave.

To check the programs, click here and look them up.

(211529)

The list is extensive for Antivirus programs and just to name a few, Total AV, Intego AV, McAfee AV, Norton AV, BitDefender and on and on the list grows.

As for the others, my favourite CleanMyMac to be removed for sure

Choose Applications from the Finder's Go menu, throw the existing installer away, and try downloading it again; you may need to go through an intermediate OS version or download it on another computer.

(211617)

No.

(211559)

It depends on what you've installed. None of them will be present unless manually added, and not all of them are bad.

(211570)