That's odd. I would assume there should be a value in 53 or 52 since those are the ones that go to line 17600 and 17599, respectively, of schedule 3, as per:
https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/tax-re...
(open reporting instructions dropdown)
This is weird, since in prior years, box 21 went directly onto line 17600, but it appears that has changed? I suppose you can verify whether its going there by looking at line 17600 and 17599 manually, but this is what why we pay for turbotax, so we don't have to go on the internet looking for answers!
I'm sorry, I'm not quite sure what to do in your case.
