Hi,
Please follow the path Extended Warehouse Management --->Master Data --->Define Storage Type
and change the Level of Avail. Qty field to 1 i.e. "Storage Bin".
This field is made blank for the cases where you want your user to decide a HU at the time of picking. If you keep this field as "1" This means that you are allowing your user to pick up any HU and system does not gives you the message you are getting.
Regards,
Rohit