Your post is unclear but it appears you sold something or something became worthless - a security(?), a loan(?), something else(?) - and you don't have a 1099-B reporting the transaction.
I don't know what TurboTax product you're using - that information can be very helpful as there are differences between versions and even differences between using TurboTax "online" vs. "desktop" - but in the desktop products, (all of which support the reporting of investment losses), you use the "Stocks, Mutual Funds, Bonds, Other" interview and TurboTax asks if you received a 1099-B. If you answer "No" you're place in an alternative interview that asks for the necessary information in order to properly report it. In the online products you need Premier or above to get into the "Stocks, Mutual Funds, Bonds, Other" interview. I assume the online product functions similarly to what I've described for the desktop versions.