If you missed adding an RRSP Contribution to a tax return you must make an adjustment to a that year's tax return.
For contributions made from Mar-Dec 2016 or in the first 60 days of 2017, an adjustment must be made to the 2016 tax return.
For contributions made in the first 60 days of 2016, an adjustment must be made to the 2015 tax return.
You can use "My Account" if you are set up, otherwise, you will need to print a paper copy and mail it to your tax centre. https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/change-your-return.html
*If you use CRA's AutoFill to import your tax information, the slip you did not claim on last year's return may import to your current year return as an "Unused RRSP Contribution". Only then would you be able to claim it on this year's tax return.
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