You didn't say what version of TurboTax you are using. It sounds like you're using the CD/Download software, and trying to enter your 1099-B in forms mode. Doing it that way is very complicated and error-prone. You should enter your investment sales in the Step-by-Step interview. If you select the option to enter a summary instead of entering each sale, TurboTax will enter the adjustment code M.
If you have further questions, please say whether you are using TurboTax Online or the CD/Download TurboTax software, and what edition (Basic, Deluxe, etc.).
