How do I report a 1099-B statement for transactions for "unknown term", basis is not reported to the IRS, and for which short-term or Long-term determination is unknown?

You can try to look back in your records for trade confirmations. Apart from that, if you can remember when it was purchased, you can at least determine long or short term. If you can't establish a basis, it defaults to zero.

You have to determine the basis and the acquisition date from your own records and enter them in TurboTax. The acquisition date will establish whether the sale was short-term or long-term.

If you inherited the investment that you sold, enter the word Inherited instead of the acquisition date. A sale of an inherited asset is always treated as long-term, no matter how long you or the deceased actually owned it.

The brokerage or financial institution might have some of the information that you need, even though they didn't put it on the 1099-B. It's worth asking.