Hi Khoram, I personally don't use lulu as a printing service (no issues with them I just use another provider) but I've had a quick look at this.
When I export the .pdf from my desktop publishing software there is an option to 'include layers' which seems to relate to the error message you've encountered.
As a test I have uploaded a new version of the Players Handbook at the link below and have not included these layers.
Would you like to see if this makes a difference? If so, then I can export the other pdf's in the same manner.
https://nameless-designer.itch.io/heroes-of-adventure-players-handbook
If anyone else has any insight on ways the .pdf should be prepared for printing then let me know.