Debian: Updating package with PPA fails with error "gpg: no valid OpenPGP data found"

My operating system is Debian 9.1 with Cinnamon 3.2.7. I installed Audacious (version 3.7.2) from the Package Manager and want to update to the latest released. Per these instructions , I attempted to update by entering the following commands: sudo add-apt-repository ppa:nilarimogard/webupd8 sudo apt update sudo apt install audacious After a prompt to press Enter, I received the following output: gpg: keybox '/tmp/tmpseyl6p36/pubring.gpg' created gpg: /tmp/tmpseyl6p36/trustdb.gpg: trustdb created gpg: key 531EE72F4C9D234C: public key "Launchpad webupd8" imported gpg: no ultimately trusted keys found gpg: Total number processed: 1 gpg: imported: 1 gpg: no valid OpenPGP data found. Exception in thread Thread-1: Traceback (most recent call last): File "/usr/lib/python3.5/threading.py", line 914, in _bootstrap_inner self.run() File "/usr/lib/python3.5/threading.py", line 862, in run self._target(*self._args, **self._kwargs) File "/usr/lib/python3/dist-packages/softwareproperties/SoftwareProperties.py", line 688, in addkey_func func(**kwargs) File "/usr/lib/python3/dist-packages/softwareproperties/ppa.py", line 386, in add_key return apsk.add_ppa_signing_key() File "/usr/lib/python3/dist-packages/softwareproperties/ppa.py", line 273, in add_ppa_signing_key cleanup(tmp_keyring_dir) File "/usr/lib/python3/dist-packages/softwareproperties/ppa.py", line 234, in cleanup shutil.rmtree(tmp_keyring_dir) File "/usr/lib/python3.5/shutil.py", line 480, in rmtree _rmtree_safe_fd(fd, path, onerror) File "/usr/lib/python3.5/shutil.py", line 438, in _rmtree_safe_fd onerror(os.unlink, fullname, sys.exc_info()) File "/usr/lib/python3.5/shutil.py", line 436, in _rmtree_safe_fd os.unlink(name, dir_fd=topfd) FileNotFoundError: [Errno 2] No such file or directory: 'S.gpg-agent.browser' Then, when I attempted to run the update step (``sudo apt-get install --only-upgrade audacious``), I received more errors: Ign:1 http://deb.debian.org/debian stretch InRelease Ign:2 http://www.scootersoftware.com bcompare4 InRelease Ign:3 http://ftp.us.debian.org/debian stretch InRelease Hit:4 http://deb.debian.org/debian stretch Release Hit:5 http://www.scootersoftware.com bcompare4 Release Hit:6 http://ftp.us.debian.org/debian stretch-updates InRelease Hit:7 http://security.debian.org/debian-security stretch/updates InRelease Hit:8 http://ftp.us.debian.org/debian stretch Release Ign:11 http://ppa.launchpad.net/alexanderk23/ppa/ubuntu artful InRelease Ign:13 http://ppa.launchpad.net/nilarimogard/webupd8/ubuntu disco InRelease Err:14 http://ppa.launchpad.net/alexanderk23/ppa/ubuntu artful Release 404 Not Found Err:15 http://ppa.launchpad.net/nilarimogard/webupd8/ubuntu disco Release 404 Not Found Reading package lists... Done E: The repository 'http://ppa.launchpad.net/alexanderk23/ppa/ubuntu artful Release' does not have a Release file. N: Updating from such a repository can't be done securely, and is therefore disabled by default. N: See apt-secure(8) manpage for repository creation and user configuration details. E: The repository 'http://ppa.launchpad.net/nilarimogard/webupd8/ubuntu disco Release' does not have a Release file. N: Updating from such a repository can't be done securely, and is therefore disabled by default. N: See apt-secure(8) manpage for repository creation and user configuration details. Finally, when I ran the install step (`sudo apt install audacious`), it said the latest was already installed. It must not have accepted the PPA. Reading package lists... Done Building dependency tree Reading state information... Done audacious is already the newest version (3.7.2-1+b1). 0 upgraded, 0 newly installed, 0 to remove and 241 not upgraded. After browsing online, I entered the following commands to undo my changes: sudo add-apt-repository --remove ppa:nilarimogard/webupd8 sudo apt update sudo apt-key del 531EE72F4C9D234C sudo apt update What additional commands do I need to run in order to get the PPA to add properly?