Exact same issue for me with Outlook (Mac 15.36.1) on MBPro running Sierra 10.12.5. It appears to install successfully. It suggests re-opening Outlook (if it was open), but I've installed with it open or close and it definitely is not doing anything at all.
Same for me. Had it running for months but now its a no go. I read a message that this app is no longer supported because its not 64bit but it didn't tell me which one. I assume its the plugin because that's the only thing not working anymore.
Same. Upgraded to Mojave and no more button to add RingCentral meeting to Outlook meeting. Is everyone else just manually creating the meeting in RingCentral and copying in the details in the meantime?