Model having XXX error / warning
Read the console. The error log walks you through the reason and the solution, and gives you additional insight on the problem itself.
Model taking damage for no reason
The "eye height" is either missing or set below 0. To fix this, bring the hitbox's bone pivot point above 0.
You can learn more about this Here
The model is made up of vanilla items!
Use the generated resource pack, located in the plugin's folder.
The plugin folder is located at
<Server Main Directory>/plugins/ModelEngine/resource pack.zip
Model using parts of a different model
Update the resource pack.