How do I enter the amount shown in Box 20 of T4A (P) slip. Turbo Tax does not provide a space and says it is not necessary, but you have to enter it as income somewhere?

The amount in Box 20 is the total of boxes 14 to 19, so once you claim the amount in one of those boxes, the program totals this for you. Most people just have an amount they need to enter in Box 14 (plus the number of months in 23)