I did all 3 steps and downloaded the lated from 11 days ago and still the same message if you create a plugin please test to make sure it works thank you
Viewing post in [RMMZ] Quest Log – Simple Quest Journal System comments
It's quite apparent that you didn't follow this 3 steps as I downloaded Core and Quest from Itch right now, put them in a vanilla project and they work. So I repeat, download Core v1.3 as that error appears only with Core 1.2 and below.
I repeat: I downloaded them from Itch, now, I didn't use my own files, and they work!