Ign http://packages.linuxmint.com tessa InRelease
Hit http://archive.ubuntu.com/ubuntu bionic InRelease
Hit http://security.ubuntu.com/ubuntu bionic-security InRelease
Hit http://archive.canonical.com/ubuntu bionic InRelease
Hit http://archive.ubuntu.com/ubuntu bionic-updates InRelease
Get: 1 http://packages.linuxmint.com tessa Release [24,1 kB]
Get: 2 http://packages.linuxmint.com tessa Release.gpg [819 B]
Hit http://archive.ubuntu.com/ubuntu bionic-backports InRelease
Ign http://packages.linuxmint.com tessa Release.gpg
Fetched 25,0 kB in 3s (8.375 B/s)
W: GPG error: http://packages.linuxmint.com tessa Release: The following signatures were invalid: BADSIG A6616109451BBBF2 Linux Mint Repository Signing Key <root@linuxmint.com>
E: The repository 'http://packages.linuxmint.com tessa Release' is not signed.
E: Failed to download some files