The drop-down is working correctly. You cannot leave the acquisition date blank on your tax return. The fact that it's blank on your 1099-B means that the brokerage doesn't know the date. You have to supply the date from your own records. The exact date isn't critical as long as you correctly determine whether the sale is short-term or long-term. If you don't know the date, you can select Various, but you can't leave it blank.
If you inherited the investment, you answered an earlier question incorrectly.
