My cashier took a sale on store credit and now the cash count is short. Did she do something wrong?

No, and the count is not short. A sale paid with store credit, a gift card or a house account does not open the drawer and does not add to the cash your staff count at closing, because no cash came in. The Z report shows each of those under its own name, next to cash and the card machine. What should be there in the drawer is the float plus cash sales plus money in, minus cash refunds and money out, and only cash. If the drawer is out after that, the difference is elsewhere. Reports → Shifts and the POS shows the same shift later with what each way of paying took.