What version of Windows are you running? Plex dropped Windows 7, 8, and 8.1 support last year, shortly after Microsoft themselves stopped supporting them*, so 1.31.3.6868 is the last version that will work with those operating systems.
* Windows still supports 2012 R2 if you have extended security updates, but as it was developed alongside Windows 8.1, Plex doesn’t support it.
