Yes, if you have disposed of (ie sold) and investment, you need to calculate the ACB of the assets in question and enter them in box 20 for the T5005.

But where is the ACB subtracted in TurboTax?

When you enter the ACB into your T5008, it then is deducted from the proceeds, in order to create the gain or loss.

I got all the way to deductions summary and net income but the ACB from T5008 line 20 had not been deducted.
I repeat my original question: where, in TurboTax, is it deducted?