This message appears when, at the point where the alert is evaluated, the data item to which it applies cannot be found using the name it had when the alert was raised.
It is possible that the target cell was removed from the dataview or renamed upon evaluation. For example, rowA was previously present in sampler 1 and in the next sample, rowA is no longer present. Also, if rowA was renamed to rowB in the next sample. These instances will trigger the message if there is an alert applied to it.