1) Have you downloaded WD_Core?
2) Have you downloaded the LATEST WD_Core?
3) Have you placed WD_Core above WD_Quest?
That way the plugin won't "suck"
Pretty sure you missed number 2 :P
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!