I had to go on to the apple website and install it from there, not sure why it hasn't been keeping up to date and I'm not very tech savvy so I just wasn't aware what the latest OS was. Thanks so much for all your help!! :)
An auto update will only happen for the minor updates of your current system version (mostly security updates and bug fixes). When Apple releases a major new release of your system (when the first number of the system version changes) and the functionality changes dramatically, you have to get the update manually. This makes sense, as you have to make sure, that the changes to the system are not critical for your current projects and you can afford the upgrade, while working on a critical project. The macOS 10.15 Catalina upgrade and macOS 11 Big Sur upgrade have made many older media formats incompatible and 32-bit applications can no longer run. Also, major system upgrades are not free updates and have to be purchased from the App Store (even if the purchase is currently free).