As long as you made contribution to RRSP, you are supposed to report it on your return. You do have the option not to claim the deduction the same year then your contributions will become "Unused RRSP contributions" available for the next year and shown on your Notice of Assessment. If you missed reporting your contributions last year, you need to make amendment to your last year's return.
For example, you made contributions in first 60 days of 2019 but forgot to report it on your 2018 return. The perfect solution would be to make a change to your 2018 return and refile it to the CRA. The CRA will reassess your 2018 return and the "Unused RRSP contributions" will be available on your 2018 re-assessment.
It is easy to make a change to your previous year's return using CRA MyAccount or reFile. Some people think it would be too late when the re-assessment is available. They just go ahead and choose a not-so-perfect solution - they just report the contributions on their 2019 tax return. It seems all good so far however in case CRA does cross-checking with banks, there is a possibility to deny your deductions.