As you discovered, you cannot make entries directly on Form 8949. All the information flows from various worksheets. Entering investment sales in forms mode is very complicated and error-prone. TurboTax strongly recommends using the Step-by-Step interview to enter investment sales. How you enter adjustments depends on the type of adjustment. What specific adjustment codes do you need to enter?
For incorrect basis (code B), on the screen where you enter the details of the sale, check the box that says "The cost basis is incorrect or missing on my 1099-B." Continue through the questions. Two screens later you will be able to enter the correct basis.
You can enter adjustments for accrued market discount (code D) or wash sale loss disallowed (code W) on the screen where you enter the details of the sale. Check the box that says "I have other boxes on my 1099-B to enter," then enter the adjustment in box 1f or 1g.
The next screen after you enter the details of the sale says "Let us know if any of these situations apply to this sale." You can enter adjustments for sales expenses (code E) or to change the holding period (code T) by checking the appropriate box on that screen.
