MacBook Air. I need Mojave 10.14

You'll, need to make a bootable USB stick...

Create a bootable installer - Apple Support

Use the App Store

To download macOS from the App Store, use a Mac that is compatible with that macOS. If your Mac isn't compatible, the App Store dims the Get button, says that the macOS is not compatible with this device, or says that the requested version of macOS is not available.

If these links don't work as expected, try again using Safari, which is in your Applications folder.

Sonoma 14

Ventura 13

Monterey 12

Big Sur 11

Catalina 10.15

Mojave 10.14

High Sierra 10.13

1. Use these links to find macOS in the App Store.
2. Cick the Get button in the App Store to begin downloading. Software Update might open and ask you to download from there.
3. After downloading to your Applications folder, the installer opens automatically. Follow the onscreen installation instructions. Or quit without installing to leave the installer in your Applications folder for later use.

How to download and install macOS - Apple Support

macOS High Sierra - Technical Specifications

General Requirements

• OS X 10.8 or later
• 2GB of memory
• 14.3GB of available storage to perform upgrade*
• Some features require an Apple ID; terms apply.
• Some features require a compatible Internet service provider; fees may apply.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen, choose About This Mac. These Mac models are compatible with macOS High Sierra:

• MacBook (Late 2009 or newer)
• MacBook Pro (Mid 2010 or newer)
• MacBook Air (Late 2010 or newer)
• Mac mini (Mid 2010 or newer)
• iMac (Late 2009 or newer)
• Mac Pro (Mid 2010 or newer)

macOS High Sierra - Technical Specifications

https://itunes.apple.com/app/macos-high-sierra/id1246284741?mt=12

macOS Mojave 10.14.6 - Technical Specifications

General Requirements

OS X 10.8 or later

2GB of memory

12.5GB of available storage (OS X El Capitan 10.11.5 or later)*

Some features require an Apple ID; terms apply.

Some features require a compatible Internet service provider; fees may apply.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen and choose About This Mac. These Mac models are compatible with macOS Mojave:

MacBook (Early 2015 or newer)

MacBook Air (Mid 2012 or newer)

MacBook Pro (Mid 2012 or newer)

Mac mini (Late 2012 or newer)

iMac (Late 2012 or newer)

iMac Pro (2017)

Mac Pro (Late 2013; Mid 2010 and Mid 2012 models with recommended Metal-capable graphics cards)

macOS Mojave - Technical Specifications

https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12

if you have access to another Mac, (beg or borrow one if necessary) one that qualifies to run either macOS, you could create a bootable installer for either macOS. also, using a bootable installer, you could go directly to Mojave without having to upgrade from High Sierra.

you can find the instructions for creating and using a bootable installer here > How to create a bootable installer for macOS – Apple Support. and the link to the older macOS can be found on that page.

That is a problem, try this...

MacBook Air SMC and PRAM reset - Apple Community

>high end hardware (photo scanner) that will only run on OS up to Mojave 10.14.

Any information about the exact scanner make and model?

Would it be correct to assume that you have a more modern Mac? If so, which model and operating system version? If at all applicable/necessary, a third-party scanner driver called VueScan (https://www.hamrick.com/) could possibly be of interest in this case.

Where does that leave us?

Oh yes, I'd be shocked if VueScan couldn't scan it! :) 👍