[Thertzler]

So the most likely issue here is that is that your command line is not actually executing in the same director as the executable.

If you are simply opening the command prompt from the start menu it tends to start in your users directory or maybe in some system32 windows folder.
It'll find the page_approximator.exe file because you provided the absolute path to it, not the epub because its path is relative.

So either set the cmd location to that folder before running the other command with this:

cd C:\Users\videv\Downloads\page_approximator_win_x64

...or use the absolute path for the book as well:

C:\Users\videv\Downloads\page_approximator_win_x64\page_approximator.exe C:\Users\videv\Downloads\page_approximator_win_x64\EssexDogs.epub 466