How to get NBT Tags from Player Inventory and so on?

Hello,

I'm trying to get NBT Tags and their value output to be stored in a String variable to later Substring it and use it elsewhere.
in a /execute Command function. 

This is the NBT function I try to use

Whenever I try to output a Message with this function it returns just nothing, SelectedItem is a valid NBT Tag for the Player, why do I not get any results, do I miss something? It drives me real crazy. Like you cant even store freshly made Entities / Blocks in the procedures after you created them or am I just stupid?