The issue is very simple, when you have de Mod Inventorio, https://github.com/Lizard-Of-Oz/Inventorio
The extra slot for the elitras is gone, you can open to remove or add elitras. I dont know if this is a issue with this or the other mod.

I believe this is an issue with Inventorio as it's been reported it doesn't work with many mods that add extra inventory slots.