Serial required in POS view ERPnext v12

Dear all
Can you help me in this problem?
When I set item with serial/batch number.
I scan in POS view. It okie, but when I pay, it has a message like that: Serial required. If I put 2 serial number, it will message : the serial NULL in stock.

When i scan 1 item and pay:

WHen i scan 2 item and pay:

They have serial number already.