The specimen was collected in the U.S. but a county is not specified on the label.
First, look again! Sometimes the county is "hidden" in the header text or somewhere else on the label where you might not immediately see it. For example, if the label header says "Plants of San Luis Obispo County" or "Flora of Fresno County", you can assume that the plant was collected in San Luis Obispo.
If you still can't find the county on the label, you can search online for the appropriate county using the locality information provided on the label. If there are multiple potential counties for a locality and you can't determine which one is correct, leave the County field blank.