Generally speaking, when you receive a invalid account access error, you are trying to perform an action using the wrong AccountID.
In short, when you first "login" (call the LocationService), you are using your NetworkAccountID. This basically gives you a certain set of rights within the API. But then as you are working in the API, this error pops up. When that happens, the API is telling you that you need to switch account contexts (i.e. change the AccountID) you are using. In the case of adding Placements, you must use your AdvertiserID of the Advertiser the Order is created for.
So change the AccountID to the AdvertiserID and you should be fine...for this operation. Think of it as having to login as the advertiser to perform certain things.