Thanks Igor. I submitted a support request and was directed that I was out of support- after using the plex migration tool to get a new instance running on the latest LTS 18 of ubuntu I was able to manually add and set the ipv6 address.
I am having a similar problem to Joost de Bruijn, I do not see IPv6 addresses listed even though I can see both IPv4 and IPv6 addresses are assigned. When I try to add the IPv6 address I am given the same error.