Please double check your T4A(P) slip and what boxes you have filled in.

Box 21 is for Number of months - disability.

Box 23 is for Number of months - retirement.

If you have something in Box 14 - Retirement benefit, then you need a number of months in Box 23.

If you have something in Box 16 - Disability benefit, then you need a number of months in Box 21.