Dear @Nathier_Abrahams - this error occurs because indeed the Launchpad PPA does not have repositories with labels for buster/stretch/jessie and other Debian series, the distribution lables in PPAs are only for Ubuntu. However, the kolibri release itself is identical for all Debian-based distros.
We have written a tutorial that was just updated for using both the
kolibri package and the
kolibri-server package, which is highly recommended to speed up the Raspberry Pi for a much greater serving capacity.
Please refer to: https://kolibri.readthedocs.io/en/release-v0.12.x/install/tutorials/rpi.html