currently running on macosmojave 10.14.6 and am attempting to update to catalina. Firstly sofware update presumes my mac is up to date. Whilst searching from the apple store, i click on get for catlina and it downloads but doesnt install

currently running on Mac os mojave 10.14.6 and am attempting to update to catalina. Firstly, the software update found on the apple, presumes my mac is up to date and therefore not upgradeable. However, whilst searching for an alternative on the apple store (online), I stumble upon Catalina and click on it. it downloads to a folder. however as it begins to install, a message pop ups saying - this update requires Mac os version 10.15