An error keeps popping up my ubuntu distribution. I am on 14.04 LTS, and this error pops up when I use Synaptic Package Manager and I click "Fix Broken Packages" so the error stops:
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://repository.spotify.com stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 13B00F1FD2C19886
W: Failed to fetch http://repository.spotify.com/dists/stable/InRelease
W: Failed to fetch http://ppa.launchpad.net/openshot.developers/ppa/ubuntu/dists/trusty/main/binary-amd64/Packages 404 Not Found
W: Failed to fetch http://ppa.launchpad.net/openshot.developers/ppa/ubuntu/dists/trusty/main/binary-i386/Packages 404 Not Found
W: Some index files failed to download. They have been ignored, or old ones used instead.
How do I fix this?