Hi there. I'm trying to install on a Raspberry Pi Desktop OS on my netbook. Output below:

$sudo apt-add-repository ppa:mutlaqja/ppa
 Latest INDI Library and drivers!

This PPA is for stable Ekos & INDI drivers releases.
 More info: https://launchpad.net/~mutlaqja/+archive/ubuntu/ppa
Press [ENTER] to continue or ctrl-c to cancel adding it

gpg: keybox '/tmp/tmpyoaw10oy/pubring.gpg' created
gpg: /tmp/tmpyoaw10oy/trustdb.gpg: trustdb created
gpg: key 487CEC2B3F33A288: public key "Launchpad INDI" imported
gpg: Total number processed: 1
gpg:               imported: 1
gpg: no valid OpenPGP data found.

Clearly something not quite right on that last line. If I try update:
$ sudo apt-get update
Get:1 http://security.debian.org stretch/updates InRelease [94.3 kB]
Hit:2 http://linux.teamviewer.com/deb stable InRelease                         
Hit:3 http://linux.teamviewer.com/deb preview InRelease                        
Get:4 http://ppa.launchpad.net/mutlaqja/ppa/ubuntu cosmic InRelease [15.4 kB]  
Hit:5 http://archive.raspberrypi.org/debian stretch InRelease                  
Ign:6 http://ftp.debian.org/debian stretch InRelease                     
Get:7 http://ftp.debian.org/debian stretch-updates InRelease [91.0 kB]
Ign:4 http://ppa.launchpad.net/mutlaqja/ppa/ubuntu cosmic InRelease
Hit:8 http://ftp.debian.org/debian stretch Release                             
Get:9 http://ppa.launchpad.net/mutlaqja/ppa/ubuntu cosmic/main amd64 Packages [10.2 kB]
Get:10 http://ppa.launchpad.net/mutlaqja/ppa/ubuntu cosmic/main i386 Packages [10.3 kB]
Get:12 http://ppa.launchpad.net/mutlaqja/ppa/ubuntu cosmic/main Translation-en [3,720 B]
Fetched 225 kB in 4s (48.2 kB/s)                    
Reading package lists... Done
W: GPG error: http://ppa.launchpad.net/mutlaqja/ppa/ubuntu cosmic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 487CEC2B3F33A288
W: The repository 'http://ppa.launchpad.net/mutlaqja/ppa/ubuntu cosmic InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.

Any ideas?

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