CO assignment missing at at the time rounding off material in MIRO

Dear All,

Iam facing the problem with co object assignment missing at the time of MIRO,

>when iam posting miro with higher value than PO, it should hit rounding off of material(GL)

>it is hitting that, but it is not allowing to post into the rounding off gl, showing error like cos object missing.

>Here we r mapped setting in OKB9 with cost centers depends upon plant wise.

>if i assign in okb9 valuation area mandatory level(1), it is not allowing me to post,

>but if i assigh GL at main screen in okb9, instead of selecting valuation area mandatory(1),it is allowing me.

Please help me how to overcome the problem, is there any special setting or logic for rounding off material gl.

>That GL has cost element, cost center is in active,field staus group setting also correct,

Where else i have to check please guide me.

Thank,

chinni