A bit over a week full of work with this. Unfortunately, it's not completed yet, I have some confusing problems with getting Fandom from the AO3 link and it seems that the script is quite slow at the moment. I'm not sure if there's anything I can do about that, but I will try.
So, first things first, what are the changes generally from the previous when thinking about how the script works?
To make everything easier, I have downloaded the whole script to Google Drive where you can copy the whole code and replace the old code with it.
1. Open your file.
2. Choose Extensions > Apps Script from the top.
It opens a new window which can be a bit overwhelming at first.
3. Choose custom menu.gs from the left side panel.
4. Delete everything from that script.
5. Go to Google Drive through this link. Copy everything. (F.e. CTRL/CMD + A and then CTRL/CMD + C)
6. Go back to the tab where you have the script open. Paste the new content.
7. Save the script as shown in the picture below.
8. Close the tab and update the file page. The Fanfiction Management menu might take a few minutes to appear.
The script has been updated also to public Log files, so you can always make a copy of it for yourself and copy & paste all the old content from the old file. However, since I'm most likely releasing version 2.1 within the next few weeks, you might want to wait for it. With the new version release I recommend strongly making a new copy and I will not give out instructions to make edits other than the complete fix for this script. If everything goes as planned, there will be a few new features I've been either working on already since before the release of 2.0 or someone has requested it within these first two months. Find the links to the file from this link.
See also the video below on how to open Apps Scripts!