When I try to download the new Mojave system on my OS X El Capitan v 10.11.6 it tells me I can't. Why is that?

Hello!

Could you let me know what model and year Mac you are using?

The issue may be that you computer is not compatible with the current operating system (macOS Mojave). Also, it could be that you have to upgrade to macOS High Sierra first then transition to macOS Mojave. Check out the following links to check the compatibility of your computer and for detailed steps on how to properly upgrade.

How To Upgrade To macOS Mojave: https://support.apple.com/en-us/HT201475

How To Upgrade To macos High Sierra: https://support.apple.com/en-us/HT208969

Let me know if this helps or if you have any further questions!

Please clarify what you mean. Are you going to be running the Windows OS using bootcamp? If so, you can find information pertaining to bootcamp by clicking the link below. Also I would recommend you get the latest Windows 10.

Boot Camp Support: https://support.apple.com/boot-camp

Let me know if this helps!

Check to make sure your applications are compatible.      

Application Compatibility

Also check to make sure there is a compatible driver for your printer.

Do a backup before installing, preferable 2 backups on 2 different drives.

True! I need to include this information when I share the update information...😂

That machine can't go any higher than El-Capitan. You've hit the limit. It is too old to upgrade any further.

Your computer is not compatible with the latest operating systems. If you would like to have access to the latest operating systems you will need to purchase a newer computer.