10.14.6

That is really your choice, but there may be compatibility issues for your device & current installations.

How to upgrade to macOS Mojave:

https://support.apple.com/en-us/HT210190

Application compatibility & feature support:

https://roaringapps.com/

[note, roaringapps.com is not hosted by Apple]

Please use Safari to access the following link 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 Mojave.app.

Move it to the Trash, empty the Trash.

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