The error message indicated that squid isn’t available within the GL.iNet repo. Nothing you can do about it. Maybe the staff will add it, or you can try to download all the necessary files from the OpenWrt repo: https://archive.openwrt.org/releases/21.02.7/
But it will be a manual process and no guarantee for anything.
Thank you very much. A “manual” installation failed; it was not the right computer architecture, if I understood the error message correctly; I used aarch64. I did not find a more in-depth specialization on the device (mediathek). Maybe I am also wrong about the processor.