Replace command greyed out in a part instance structure tree

Hi,

I’m trying to make the “Replace” button active in a structure of a Part alternate:

I can’t understand why it is not active. Looking at the following documentation https://support.ptc.com/help/windchill/r13.1.2.0/en/index.html#page/Windchill_Help_Center/configsandinstances/CIInstanceReplacePartWithin.html I don’t see what is justifying this behavior. I already have an Alternate for the Part I want to replace.

Any ideas?

Cheers,

Yoan.