First of all, I want to make sure you ACTUALLY want to use Plastic Animation Paper, which is an old predecessor to Animation Paper? Animation Paper is MUCH better and robust even though it is still under development. So sign up for the alpha pre-release of Animation Paper here: https://animationpaper.com/#HowMuch – it is free too. You will receive the download link right away.
PAP is 25 years old – last line of code was written over 15 years ago. So if it works on modern systems it is amazing and frankly pure luck. 😉 But about the problem you ask about: Did you check if the projects_path file is pointing correctly? Alternatively you can just install it again, to make it set up correctly automatically.
But again, just drop PAP and get AP! 😉