Three of Apple's Automator actions call on python2, despite Apple removing it. They won't run under python3 without modification.
1. Install python2.
The scripts call
env python, so you can install python2 direct from python.org (you'll also need to
2. Use Apple's own Quick Action "Create PDF" to Combine PDFs.
This works well for combining PDFs in the Finder, but if you want to use the action as part of an Automator workflow, consider Shortcuts.app instead, whose PDF actions don't use python.
3. Use an updated python3 script for PDF manipulation.
I've produced (better) python3 scripts for a range of PDF manipulation, including concatenation, and a grid.
You can include them in Automator or Shortcut.app's Run Shell Script action, which offers any installed languages. (Select "Pass Input as Arguments")
Again, if you install python3, you'll need to
pip3 pyobjc to get the necessary ObjC bridge library.