I can't update my Mac from macOS High Sierra

The model year of a MacBook Pro is available at Apple menu > About This Mac.

An Early 2008 through mid 2009 MacBook Pro can be upgraded to El Capitan 10.11.6 but no later version. See: https://support.apple.com/en-us/HT211683

A mid 2010 through late 2011 MacBook Pro can be upgraded to High Sierra 10.13.6 but no later version. See: https://support.apple.com/en-us/HT211683

A mid 2012 through early 2013 MacBook Pro can be upgraded to Catalina 10.15.7 but no later. See: https://support.apple.com/en-us/HT211683

A late 2013 and mid 2014 can be upgraded to the latest  version of Big Sur macOS 11 but no later. See: https://support.apple.com/en-us/HT211683

An early 2015 or later can be upgraded to the current version of Monterey macOS 12. See: https://support.apple.com/en-us/HT212735

What year MacBook Pro do you have? You need MacBook Pro introduced in 2015 or later

Is MacOS Monterey available in the App Store?

Some users have experienced problems when skipping macOS versions. I recomment that you update incrementaly. To get the older version

How to get old versions of macOS - Apple Support

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

Tips Monterey Upgrade  Released Oct 25, 2021

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

Moving to a newer OS version is not an update. Apple calls it an upgrade and those do not show up in Software Update. You have to get the most current OS from the Mac App Store.

If your Mac cannot run that OS, you can upgrade to one of the others from here: How to get old versions of macOS - Apple Support

That is not true with the later macOS versions. Later ones like BigSur will list the Monterey upgrade is Software Update but you have to click to install upgrade.

Barney-15E wrote:

Moving to a newer OS version is not an update. Apple calls it an upgrade and those do not show up in Software Update. You have to get the most current OS from the Mac App Store.

I don't have any old OS's to compare to, but that Upgrade notice is only available for about a month after the release of the new OS. And, I don't imagine it would appear for anything but the most recent previous OS, being Big Sur. My only Catalina Mac couldn't run Big Sur, so I don't have anything from which to compare.

The listing of an Upgrade from older versions to newer version was introduce in Mojave macOS 10.14. The OP specifically denotes Sierra macOS 10.12.

Therefore - it would not be listed