Is the Windows Media Player installed and not disabled?
Is ‘Internet Explorer 11 for Windows 7’ installed?
Are all available Windows Updates installed? (may need several update/reboot cycles)
Hint: Windows 7 will be ‘unsupported’ by Microsoft in January 2020.
Plex Server will very likely cease to support Windows 7 at the same date (or shortly thereafter). Now is the time to think about a migration plan.