Mac will not update

According to your tag line, you have a 13" MacBook Air that is running macOS 10.14 (Mojave).

Any MacBook Air that is compatible with Mojave can be upgraded to a later version of macOS. How far you can upgrade would be the question, and we cannot answer that without your MacBook Air's hardware model year as shown in About This Mac ( > About This Mac…).

If your hardware model year is

• Mid 2012 - you can upgrade as far as macOS 10.15 (Catalina). While that's not very far, it would be enough to allow you to run the current version of Firefox. Note that Catalina and later break 32-bit applications.
• Mid 2013 or Early 2014 - you can upgrade as far as macOS 11 (Big Sur).
• Early 2015 or 2017 - you can upgrade as far as macOS 12 (Monterey).
• 2018 or 2019 - you can upgrade as far as macOS 14 (Sonoma). Sonoma is still one of the "most recent three", although it isn't enough to run the latest versions of {Pages, Numbers, Keynote}, and will fall out of the "most recent three" when Apple releases macOS 27 (Golden Gate) this fall.

How to download and install macOS - Apple Support

The Age of this computer will Govern what newer version of macOS it will Qualify to install. 

Find out which macOS your Mac is using - Apple Support (CA)

Does this computer appear as Listed and qualifying for an upGrade to a newer version of macOS ? 

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.

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

If Yes, suggest having at least 50 GB of Empty Space on the drive.

It will allow the Full Version ( about 12.xx GB ) to download, then expand and then commence to install.

If there is Third Party Security Software installed - remove it.

It will interfere with both the download and installation process.

The below links will help identify which version of macOS  this computer may Qualifies to run.

macOS 27 Golden Gate  

macOS Tahoe is compatible with these devices.

macOS Sequoia compatible Computers 

macOS Sonoma is compatible with these computers

Computer Qualifying for macOS 13 Ventura 

Always make a Time Machine Backup  before doing any upDates / upGrades 

If I'm not mistaken, your MacBook Air is from mid-2012, is that correct?

And macOS 10.14 is macOS Mojave.

I'm sorry, it is no longer possible to update your MacBook Air.

Here is a 27" late 2012 iMac with macOS Catalina; I can't update that either. But I can still use it well for daily use.