No the functions of the add-on are all free (although our capacity to provide individual support is limited for free users)
I tried and I can’t reproduce the error. Creating a copy of a document is a one line operation on our end. If that fails it’s on Google’s side. We’ve seen temporary issues before, that’s why it makes sense to try again later.
Also, there might be some problem with the particular document which prevents it being copied. Unfortunately we’ve encountered a few bugs in Google’s AppScript system and I can’t rule out that’s yet another.