Can I have a QR code at the counter for walk-ins, not tied to a table?

Two ways. Tick Print a code to your ordering page under Settings → Printing & receipts and every receipt carries it, which is the cheapest QR ordering system there is. Or add a table called Counter under Settings → Tables & QR ordering and print its card for the counter; a scan then opens your menu with Counter as the table, and the kitchen ticket says so. Your normal link never offers table ordering, so a counter card is the way to give walk-ins the seated experience. The Dine-in, QR & group orders tab in the docs covers table QR codes.