Edit the Cost Center Hierarchy

Hello,

I have a very stupid problem. I try to add some new nodes on my standard cost center hierarchy. And the menu is greyed out. I can't understand why.

We have tried with a different user; we have checked the roles (SAP_BR_OVERHEAD_ACCOUNTANT, unrestricted for write & read) ; we have tried in the P and the Q-System...

Can somebody give me a hint ?

Thanks in advance,