I was just wondering if the CustomAdvancements mod was server-side, or if it is both client and server-side?

Hi, thanks for reaching out. The mod is required on both sides, client and server side to work properly. Please note: when installing on a server the directory in which the json files for the advancements need to be placed is located inside your servers directory and not inside your clients directory.
Hope that helps.

Thank you for the info and reply! It does help :)