Hi there - and thanks for reaching out. Sorry that you're struggling with this issue. It is hard to tell from the brief error message what the problem is. However, I know that the program has changed significantly since version 2.7.17 which was published back in February!
So I would highly recommend updating to the latest version 2.9.7 (available here) and trying again. If that still fails, please use the "Create Archive" button on the Help tab of PTXprint, to make a ZIP file which you can send to us (ptxprint_support @ sil.org) - that will help us replicate the issue locally and hopeful help diagnose what is breaking it.