Inventory Document not activated error

Hi Friends,

In My scenario, user has created the Inventory document in WM, and he has counted it and he is trying to Finalize the quantity difference in LI20 at WM level there User is getting error as "Inventory XXXX is not yet activated", and in further i have seen in LI03 transaction for the given Inventory document and found the document is not activated, Folks please guide me in this issue and how to activate this document ?

Thanks in advance