If you have Windows N or KN edition (you can check it by pressing Win+R, typing winver and hitting enter) you need to install Media Feature Pack. Please follow one of the links below.
- Windows 8 and later - https://support.microsoft.com/en-us/help/3145500/media-feature-pack-list-for-windows-n-editions
- Windows 7 SP1 - https://www.microsoft.com/en-us/download/details.aspx?id=16546
The solution applies if "mfplat.dll is missing" can be found in vridge.log or clientLogs.txt (Riftcat/logs).