Feel free to share where the rpaExtract.exe is and where the .rpa file is in detail. Without knowing that I would guess:
1. .rpa file and rpaExtract.exe are on different drives -> move the rpaExtract.exe to the same folder as the .rpa file
2. the .rpa file is inside a .zip or .exe file and was dragged onto the rpaExtract.exe file -> extract the game in full before dragging the .rpa file onto rpaExtract.exe
3. the path contains non-ASCII characters -> move .rpa file and rpaExtract.exe to something simple like D:\rpaex\ and try there.
If nothing helps, which version are you extracting? The latest from the website works.