The Disability Tax Credit is a CRA tax credit (not a deduction). It is not directly related to CPP Disability payments.
If you are receiving CPP Retirement benefits for 12 months in 2018, you should report these from box 14 of your T4A(P) and enter 12 from box 23.
You should not enter anything in box 21,
Your DTC amount is entered elsewhere on your return. It never appears anywhere on a T4A((P) because it is nothing to do with CPP.
