can I update iWork 09 to iWork 10 on OSX 10.11

If your iWork '09 suite was installed from the retail iWork '09 DVD, it would be a good idea to update it with the last Apple update for it in the iWork Update 9.3. This gets you the last application versions for iWork '09 with autosave and version support. It works just fine on macOS El Capitan.

Apple dropped the iWork branding when they released the newly rewritten Pages, Numbers, and Keynote in Oct 2013. These are known by their application name and version (.e.g. Pages v5.6.2). These applications still in 2022 do not support all of the features you would have in the iWork '09 applications, and because they are in different document formats, cannot be opened with the iWork '09 applications. These updated iWork '09 applications are in many ways, superior to current counterpart versions.

On macOS El Capitan, you can launch the Mac App Store and sign-in with your Apple ID. Press and hold the option key while you click on the Purchased toolbar item. If Pages appears there with an Update/Install button, click that and see what happens. It may first tell you that you need to upgrade macOS to install Pages, followed by another dialog to install the last compatible version for El Capitan. In either case, do not click Cancel, and you may end up with Pages v5.6.2 for El Capitan. If that works, repeat the process for Numbers and Keynote.