Upgrading os version

Any particular errors or diagnostics when it fails?

What have you tried?

Here are the Mojave requirements:

How to upgrade to macOS Mojave - Apple Support

If your MacBook Air meets those and has suffiicent memory and free storage, it should upgrade to Mojave.

If the upgrade to Mojave is failing, then there’s probably some software blocking this (anti-malware and anti-virus and network-monitoring-and-firewall add-ons can cause weirdness) or some other insufficiency or some corruption that’s blocking the upgrade. If that’s the case, sometimes making an external backup or two uisng Time Machine or a disk image to external storage and a bootable Mojave installer can be used. Create the external backups, create the bootable installer, boot the installer, wipe the internal disk, install Mojave, and migrate in the contents of the backups using the Migration Assistant tool.

Could you tell me how you know you know that you can't?

I recently helped out family upgrading to High Sierra which showed in the AppStore as having been 'Installed', but the upgrade didn't seem to work. I went out of my way to perform a clean install of their original OS version and then upgrade while probably all I really had to do was to go to 'Applications' and find the 'install High Sierra' app (it was called something like that).

If it is the same with you, all you have to do is to fire up that application.

On older Macs, I would really suggest to do a clean install. I even noticed a speed difference on my newer iMac and MacBook Pro. It is more time-consuming, but it was definitely worth it to me.

https://support.apple.com/kb/SP777?locale=en_US