What unofficial app are you trying to install?
Have you performed a subforum search via the magnifying glass icon and reviewed the several other discussions on how to potentially deal with a rootfs full error message? Here are just two discussions that deal with trying to fix or solve the rootfs full issue:
https://community.wd.com/t/how-to-free-some-rootfs-space-without-lossing-my-data/95555
https://community.wd.com/t/rootfs-is-full-how-do-i-remove-delete-files-from-drive/92065
