Get Numbers & Pages et al. on macOS 10.11 El Capitan
I had this issue too. Whenever I used an Apple ID that hadn't previously downloaded iWork I wasn't able to download the free version on 10.11. I even tried this workaround.

What did work was getting access to a 10.12 Mac. Downloading Pages, Numbers and Keynote. The apps were now associated to that Apple ID. I then went back to the 10.11 Mac's, signed in using the same ID and clicked download. The App Store displayed a message asking if I would like to download an older compatible version.