Basically mongosk loads after plugins. Essentials then fawe loads and that makes mongosk not work. Many people have this issue.
I talked with the owner of mongosk and he made this update to fix that along with other features.
Hello mongosk released a new update
Basically its been broken and this is a potential fix. Please add it soon.
https://github.com/Romitou/MongoSK/releases/tag/2.0.3 here is the link to it