Assistance with latest OS X online download

Sometimes you may be required to first install macOS 10.11 El Capitan when upgrading from much older versions of macOS. You may want to create a bootable macOS El Capitan USB installer.

https://support.apple.com/en-us/HT201372

Make sure your computer meets the minimum system requirement for the OS you want to install. The requirements will be shown when you view the particular instructions when access the macOS details page for each OS.

How to upgrade to macOS Catalina

Although macOS 10.15 "Catalina" is the latest version, there are links in that document that explain how to obtain earlier versions. Refer to the following, under "Learn More":

• For the strongest security and latest features, upgrade to macOS Catalina. If you have hardware or software that isn't compatible with Catalina, you might be able to install an earlier macOS, such as Mojave, High Sierra, Sierra, or El Capitan.