You cannot enter an adjustment amount without an adjustment code, but TurboTax will not allow you to enter the correct adjustment code for 1099-B income received as a nominee, which is code N.
TurboTax "does not support reporting a transaction for which you received a Form 1099-B or 1099-S as a nominee." You cannot use TurboTax to prepare your tax return in this situation. This limitation is listed in "Calculations Not Supported in the 2015 TurboTax Individual Federal Tax Program." See the FAQ at the following link.
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In the FAQ, click on the "2015" link to download the complete list of calculations not supported. Nominee dispositions is item 29 under Income, on page 4.
