Hi Santosh,
Check Packaging Material Types used for "pack_box" in MM02.
Check >> SPRO>> Logistics - General >>Handling Unit Management>>Basics>>Define Packaging Material Types>>
Check then Number assignment "Number range interval for "HU_VEKP"
Maintain
Int. interval 01
Ext. interval 02
And then maintain no range accordingly....
Then put pack_box mat in del and now Hu no should take automatically.
Regards
Suraj