As of writing (6/16/2021) the Import All Animations option in Pokemon Essentials is broken, this is good for creating a new PkmnAnimations.rxdata file when you've got a lot of animations to import
Download from releases, make sure the folder you put the exe in has these folders: Graphics/Animations (Animations folder in a Graphics folder), Audio/SE (SE folder in an Audio folder), and Data
Also make sure you have your Animations folder you've made from Pokemon Essentials' Export All option (that one isn't broken)
(You might need Ruby installed for this to work, you can get it here: https://www.ruby-lang.org/en/downloads/)
Don't forget to Export your old Animations (along with the ones you want to add) before replacing your PkmnAnimations file.