Cant update my mac os-

I think I can help you. It is important to understand the difference between updates and upgrades. An update is when we install a newer version of the same operating system. It will be found in the System Preferences by clicking the Software Update button. If you already have the latest version installed, you will be notified that no update is available. An upgrade is when you install a newer  operating system such as upgrading from Catalina to Big Sur. What you can install is dependent on the year and model Mac you have. Upgrade links may be found in the Apple App Store. Or you can click on the links below. You should only use Safari for these downloads, never Firefox. I hope this information is helpful in clearing up any confusion.

DOWNLOAD:

High Sierra, Catalina, or 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.  For Best results use Safari to commence the download as Others may not work.

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

Tips Big Sir Upgrade Released Nov 2020. This computer may qualify. Be certain to check the Empty Space Requirements.

Tips Monterey Upgrade  Released Oct 25, 2021 This computer may qualify. Be certain to check the Empty Space Requirements

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.

Suggest using Go64 to find 32 bit application before attempting the upGrade. Some or many of the Application already installed may required update or upGrades to 64 Bit Versions.

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