zmclearan is incorrect about this.
If your Mac is running Mac OS X 10.11.6, El Capitan, the recent version of iTunes is supported on this version of the Mac OS, which will update your iPad to the latest iPadOS.
If you still have issue using the latest version of iTunes with OS X 10.11.6 El Capitan, , this older version of iTunes is also still supported with iPadOS.
Here is a link to the older 12.6.5 version for download found in this Apple weblink.
iTunes 12.6.5 found here.
https://support.apple.com/en-gb/HT208079
iTunes 12.6.5 for the Mac requires OS X El Capitan
I have used both of these versions to update my wife's iPad to iPadOS and all my iMacs are running OS X 10.11.6 El Capitan.
I had NO issues.
I hope this reaches you and this works for you as it has worked for me.
Best of Luck to You!