Hoping only that file is missing, I uploaded the ffmpeg.exe used in the RC to my cloud, you can download it from here:
If that does not fix your issue, I would download the portable version of Krita’s RC and copy over its bin -folder to the one of your installation. Choose to skip existing files instead of overwriting them, this way you should get all missing files.
Here is the direct-link to the Krita 5.2 RC1:
Edit:
The RC is no longer hosted at that URL, so please go for the recent release-versions of Krita or Krita Plus and Krita Next, if you really need the RC1 I can upload the Windows setup-version to my cloud on request:
Michelist