Account XXX requires an assignment to CO object

Hi Experts,

I'm having trouble in resolving the error while doing MIRO " Account XXX requires an assignment to CO object". I went to OKB9 and checked that the required CCnt is already assigned to the particular CElmt (there are 4 CCnt assigned to 1 CElmt with different Plants). User is unable to put the CCnt in MIRO. The field is grayed out. Please suggest.