Where do I enter capital losses on turbotax 2019? It will not let me enter a negative number in the "Capital gains" tab.

A capital loss is calculated by entering the "Proceeds of Disposition" (cash received on sale) and the ACB/Book Value (cost to purchase the shares/fund units) and the negative difference will be reported as a Net Capital Loss. The easiest way to accomplish this is by using reporting slip T5008.