parameters on tree

Hi everyone,

i'm using Assembly Design in 3dexperience, i want to visualize a lenght relation as parameter in the "parameters" folder that appear in the tree. 

I have problem because when i open the "parameters and relations" function ( f(x) ) and i try to create a new parameter i recieve the following erorr:

So, i cannot create a parameters folder in the tree.

Can someone help me for create that folder and how put inside my lenght relation? 

 

Thank you very much!

 

Edu Assembly Design