As you can see from the log I'm getting the warning
This is because in Dissertate.cls, the following line is present:
Package quotchap takes the palatino option and tries to use font ppl to write chapter numbers at the start of each chapter. I have installed the Palatino Linotype font family by copying files from a Windows installation, and since xetex is used to compile the template it should be able to use all fonts installed on my system (this is just my guess). For some reason though the font is not found, unlike EB Garamond which I also installed specifically to be able to compile this template. What am I doing wrong?