My iMac is not compatible with the latest version of the OS. I currently have High Sierra. This page says Mojave and Catalina are compatible, but when I follow the links to download them it says they are not available in the store.
My iMac is not compatible with the latest OS but I need to upgrade to at least Mojave. iTunes says it's not available in the store. How can I upgrade to a version older than the current version?
iMac 27″, macOS 10.13
