I need to enter my RRSP contribution for the first 60 days of 2018, in my 2018 tax return... but Turbotax doesn't have that contribution period as an option!

To report any contributions you may have missed from the First 60 Days of 2018, CRA requires you to report them on your 2017 tax return. You'll need to make an adjustment to the 2017 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.

If you use CRA's Auto-Fill service to import your tax slips, these contributions may be in CRA's records and downloaded to your return as a "carried forward contribution" which can be claimed on the current year tax return. 

https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/change-your-return.html 

Thanks! I actually checked my previous return and it looks like I somehow covered that amount I was missing. Otherwise, you are right, I would have had to file a change to previous return.