"Can I deduct the missing rent payments?"

No, you cannot deduct revenue that you never received or previously reported as income.

Yes, you can still deduct all your expenses, even though you didn't receive all the rent. This will result in a reportable loss.

But, as the other replies also indicate, you cannot deduct the unpaid rent.

Understand that both @tagteam and @Hal_Al are right. I know I first I was perplexed (for about a tenth of a second) with one saying no and the other saying yes.

NO, you can not deduct rental income that you never received or paid taxes on in the first place.

YES, you will still claim your rental expenses.

The unpaid rent is *NOT* an expense since it's not an expense that you physically took money out of your pocket and paid to someone other than yourself. You will report in the rental income section only the amount of rent you "actually received" in 2019. That's it.