Permissions are a minefield. Peoples PC's have so many different setups and its even worse if its a Business or School's PC!
Like Bassquake said, try running the app as Administrator. The location of the file looks like it should work ok.
I see you've targeted a OneDrive location for the mp4. Sometimes using cloud based sources can have a unwanted effects when running programs, but in your case it should be fine as its only the source of the video and your working directory (output) is on your PC. Once ffmpeg has extracted the frames & file information, DainApp shouldn't need to query the source file.
I hope this works for you,
Theo