Hi Kamlesh,
May I Know why do you want to create Inventory GL as cost element?. Generally Balance sheet accounts will be created as cost elements only in Asset recon accounts and in case of valuated sales order stock, where such inventory account shall be created with cost element category 90 which is statistical.
This category is automatically assigned by the system when you create cost element in co. Otherwise balance sheet gl cannot be created as Cost element.
BR
Karthik