toast@waffle:~$ sudo apt-add-repository 'deb https://dl.winehq.org/wine-builds/ubuntu/ bionic main'
[sudo] password for toast:
toast@waffle:~$ sudo apt-get update
Hit:1 http://linux.teamviewer.com/deb stable InRelease
Hit:2 http://linux.teamviewer.com/deb preview InRelease
Get:3 https://dl.winehq.org/wine-builds/ubuntu bionic InRelease [4,701 B]
Get:4 http://security.ubuntu.com/ubuntu bionic-security InRelease [83.2 kB]
Hit:5 http://archive.ubuntu.com/ubuntu bionic InRelease
Hit:6 http://archive.canonical.com/ubuntu bionic InRelease
Err:3 https://dl.winehq.org/wine-builds/ubuntu bionic InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 818A435C5FCBF54A
Get:7 http://archive.ubuntu.com/ubuntu bionic-updates InRelease [88.7 kB]
Get:8 http://archive.ubuntu.com/ubuntu bionic-backports InRelease [74.6 kB]
Ign:9 http://packages.linuxmint.com tara InRelease
Hit:10 http://packages.linuxmint.com tara Release
Reading package lists... Done
W: GPG error: https://dl.winehq.org/wine-builds/ubuntu bionic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 818A435C5FCBF54A
E: The repository 'https://dl.winehq.org/wine-builds/ubuntu bionic 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(8) manpage for repository creation and user configuration details.
W: GPG error: https://dl.winehq.org/wine-builds/ubuntu bionic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 76F1A20FF987672F
E: The repository 'https://dl.winehq.org/wine-builds/ubuntu bionic InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.