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
Caution - jumping over all the newer versions and directly to Monterey on Qualifying computers is Technically possible - there are those who have attempted and were successful.
Then there are those who attempted and had less than good results.
If this is a Production or Mission Critical Set-up - a more prudent and measure approach is suggested.
The above is from observations on the Apple Support Community Forums and IMHO
Make Time Machine Backup is very useful and can be used to Revert to Previous Working macOS
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
