I want to insert cross reference but “cross reference” item in menu “Insert” is inactive (it’s grey). What may be reason for this?
Is your cursor “free” in the document? As in, nothing is selected or anything, and you are not in a table?
… or a frame. +1
it looks like it’s duplicate of Libre office Menu accelerator keys not working
No I don’t think it is, unless you have written your question inaccurately. A greyed out menu means that option is not available in the current context. The question you link to deals with a separate issue relating to the global Unity menu not being accessible via keyboard acceleration i.e., Alt+F for File etc.