I think you are confusing the different slips. The
T4A doesn’t have a box 21 or 23. Those are on the T4A(P), and they are
required. You must put the number of months in box 23 if you got a retirement
benefit (box 14). You must put the number of months that you were disabled if
you received disability benefits (box 16). If you don’t have them on your slip,
then the slip has a mistake. You can contact the CCP and get an amended one: https://www.canada.ca/en/employment-social-development/corporate/contact/cpp.html
You do not need to enter the amount that appears
in Box 20 of your T4A(P).
