I have unclaimed RRSP deductions from the 2015 tax year. How do I claim that deduction on my 2017 income tax return?

You must declare the RRSP contribution in the tax year you made the contribution. If the contribution is for tax year 2015, and you've already filed your 2015 return, you will need to submit a T1-Adj for your 2015 return in order to "claim" it.

https://turbotax.community.intuit.ca/replies/3404101