The error QT Application not found or missing will appear at launch of VideoStudio Ultimate 2021.
File mismatch in qwindows.dll pertaining to the Bonus Feature filters from NewBlue FX.
Once VideoStudio is installed / reinstalled, open Apps in Features from Windows Settings, then locate and remove any NewBlue entries found. If these are not present, navigate to C:\Program Files\NewBlueFX\uninstallers and run the uninstallers from here. Reboot the computer when finished, and try the launch again.