My 1099-B does not list the cost basis for Short-Term Basis Not Reported to IRS (Noncovered). But I cannot skip this box. What now?

Your financial institution may report transactions where one or more data elements are unknown by the institution and cannot be reported.

In such a case, you have to determine cost basis from your own records.  If you purchased the investment, the cost basis is what you paid for it.  

If you inherited the investment, the cost basis is the market value on the date of death of the person that you inherited it from.

If you received the investment as a gift, your basis is the donor's basis.

In some cases, you may be able to research these items by using a tool returned under a Google search for 'historical stock price lookup tool'.

See also this TurboTax Help.