Which Plex add-on are you using? The one from the Kodi repo doesn't work on the latest Kodi versions.
I use the one from the Plex forum and it is superb, having started out as an update to the broken Kodi add-on. It's now so much more. Hope this helps
https://forums.plex.tv/t/pm4k-plexmod-for-kodi-18-19-20-21/481208