How do I upgrade from 10.14.6 to 10.15

The Combo Updater you attempted to use works only with systems from 10.15 or later. You first need to get your Mac to Catalina 10.15

You can access the link for the Catalina installer here in Apple's page called How to get old versions of macOS. Use Safari to click on the link for Catalina and this will generate the Catalina page in the Mac App Store.

Download it from the App store and run the installer.

Then apply the Catalina combo update.

The usual caveats apply for installing upgrades which are well documented in the forums: make sure your Mac is capable of running the OS and backup everything before you install.

Is there a reason you do not wish to install Big Sur instead? A MacBook Air from 2013 or later will support Big Sur.

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. Make sure to use Only Safari to commence the download as Other may not work. This will download the Full Installer required to install

Make a Time Machine Backup Before attempting. Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina and Big Sur Only support 64 bit and there is not workaround.

Are there any AntiVirus, Disk Cleaner, Optimizers, Defrag, VPNs 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 click on Catalina.

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