Update OS Problem.

nym84 Said:

"Update OS Problem.: Hello, i have macbook air with Macos High Sierra, i can't update the os with upcoming note "This version of macOS 10.15.6 cannot be installed on this computer" .. how to fix it?"

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IncompatibilityMay be the Culprit:

So, verify the compatibility of macOS 10.15.6 on your Mac: Go Here: macOS Catalina is Compatible with these Computers - Apple Support. If it is incompatible with your Mac, either upgrade to a much newer Mac, or just stick with what you have.

Also...

Backup Prior to Installation:

Create a Time Machine Backup, that way, you can have something to restore your Mac from, should anything go wrong with the upgrade. Use an external hard drive that is at least 1.5 the total storage capacity as that of your internal hard drive.

Security Software:

As for security software, get and use only Malwarebytes for Mac. This is software that searches for malware/adware, with may be making this not install. So, scan with it every now and then, and remove what is found from the quarantine. Once removed, restart the Mac. It is created by longtime users of these forums. So, that is why it is the only way to go about securing your Mac, when it comes through use of software.

Downloads:

1. Malwarebytes Anti-Malware for Mac
2. Malwarebytes uninstaller

The model and year of your Mac will determine which macOS you can upgrade to,

only macs from 2012 can upgrade to macOS Mojave or macOS Catalina.

Please use Safari to access the following links do not use Firefox

How to upgrade to macOS Mojave - Apple Support

Before upgrading make a full back up of your Mac.

Check that third party apps you use have updates available for the new OS.

https://roaringapps.com

Check that your printers/ scanners etc have updated drivers and software available for the new OS.

Connect your mac to your router via cable rather than rely on WiFi.

Do not use your mac while it is downloading

and do not use any other devices that share the same internet connection.

If the App Store says ‘Open’ then it is detecting a previous download of the install app

go to the Applications folder locate Install macOS ******.app.

Move it to the Trash, empty the Trash.

Restart, click on the link again, the App Store should say ‘Get' or 'Download'.

If you are considering upgrading to Catalina be warned that it does not support 32bit apps.

Your Mac is probably running 32bit apps and these will need to be updated to 64bit apps,

some updates may be free, some may be paid. Use this app to discover which apps you have

are 32bit, https://www.stclairsoft.com/Go64/. Once the list has been created you can then

contact the developers of the apps to see if they provide 64bit updates. 

How to upgrade to macOS Catalina – Apple Support