This Medicare error can mean one of two things. Either Medicare thinks you have used the wrong assist item 51300/51303 based on the surgeon's items, OR the surgeon has not yet submitted her/his claim and the operator has rejected your claim.
Firstly check that you have used the same item numbers as the surgeon. Next, check if the surgeon's rooms have already submitted a claim. Once that is done, resubmit the claim, and it should be okay.