I too am having problems downloading and installing indi/kstars. I hav eadded the mutlaqja repository but when I attempt:

sudo apt-get update I get the following response and a refusal to download any files. I'm using Linux Mint 20.

W: GPG error: ppa.launchpad.net/mutlaqja/ppa/ubuntu focal InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 487CEC2B3F33A288
E: The repository 'ppa.launchpad.net/mutlaqja/ppa/ubuntu focal InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure( manpage for repository creation and user configuration details.

Help would be gratefully received.

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