Can you manually input the IP address in your browser followed by :32400/web instead of using the Configure button? Is your NAS on the same network (subnet) as where ever you’re browsing from?
Please double check that your browser is allowing local connections, see: Important note about the Plex Web app - Local Network Access
Please download the latest PMS version here: https://www.plex.tv/media-server-downloads/?cat=nas&plat=wd
It might be helpful to review this other forum thread: https://forums.plex.tv/t/wd-mycloud-server-inaccessible-after-installing-latest-update/940866
