What did you enter for the investment type?  Stock, Personal Item, etc.

When entering the sale, was the cost basis correct?  Did you check the box the cost basis was missing or incorrect?

Did you import the 1099-B from your brokerage account?  If you did the cost basis may not have transferred.  You can check Form 1099-B and manually enter the cost basis following these steps.

•  With TurboTax open enter form 1099b  in the search box.
• Then select Jump to  1099-B in the results window.
• Any Form 1099-B imported will show up on the next page Here are all your investment sales
• If your 1099-B is listed select edit to add your cost basis
• If your 1099-B is not listed follow the prompts to enter your Form 1099-B sale information.