Also having this error 
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://downloads.plex.tv/repo/deb public Release: The following signatures were invalid: BADSIG 97203C7B3ADCA79D Plex Inc.
W: Failed to fetch https://downloads.plex.tv/repo/deb/dists/public/Release.gpg The following signatures were invalid: BADSIG 97203C7B3ADCA79D Plex Inc.
W: Some index files failed to download. They have been ignored, or old ones used instead.
I’ve tried several iterations of commands in this thread… console keeps saying OK… but apt update still producing the error.
apt-key list shows this:
pub rsa4096 2015-03-22 [SC]
CD66 5CBA 0E2F 88B7 373F 7CB9 9720 3C7B 3ADC A79D
uid [ unknown] Plex Inc.
sub rsa4096 2015-03-22 [E]
