Update Failure Spotify key

[BradKlassen]

===========================
Install Updates Error log
===========================
Install Updates could not fetch the package cache information lists.
Go to https://www.freecinema2022.gq/forums/ and paste the log below into a new or existing thread for assistance.

============ Log ===========

Hit:1 http://us.archive.ubuntu.com/ubuntu bionic InRelease
Ign:2 http://dl.google.com/linux/chrome/deb stable InRelease
Get:3 http://us.archive.ubuntu.com/ubuntu bionic-security InRelease [83.2 kB]
Hit:4 http://dl.google.com/linux/chrome/deb stable Release
Hit:5 http://ppa.launchpad.net/otto-kesselgulasch/gimp/ubuntu bionic InRelease
Hit:6 http://archive.canonical.com/ubuntu bionic InRelease
Get:7 http://repository.spotify.com stable InRelease [3,302 B]
Get:8 http://us.archive.ubuntu.com/ubuntu bionic-updates InRelease [88.7 kB]
Get:10 http://us.archive.ubuntu.com/ubuntu bionic-backports InRelease [74.6 kB]
Hit:11 http://ppa.launchpad.net/teejee2008/ppa/ubuntu bionic InRelease
Err:7 http://repository.spotify.com stable InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A87FF9DF48BF1C90
Hit:12 http://repo.linuxliteos.com/linuxlite diamond InRelease
Reading package lists...
W: GPG error: http://repository.spotify.com stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A87FF9DF48BF1C90
E: The repository 'http://repository.spotify.com stable InRelease' is not signed.