On your T5008 slip, a capital loss occurs if the amount in box 20 is greater than the amount in box 21, (signaling the cost or book value associated with the securities transaction is greater than the proceeds of disposition or settlement amount).
If you enter any T5008 slip with a capital loss into TurboTax, you are given a warning when you reach the Review stage. If you are sure that you have entered your T5008 amounts correctly, you can ignore the warning. You are also given the option of going back and verify your amounts before continuing with the review.
To deal with the warning related to capital loss: