Where do I enter my 2023 certificate of rent paid if I did not complete it last year?

You cannot use a 2023 CRP on a 2024 tax return, if that is what you are asking to do.   You have not mentioned what state you are in.   You might need to amend your 2023 state return to include the CRP or you might be able to submit it directly to the state---depending on what state it is.

I am in Minnesota 

If you filed your 2023 return, you need to amend it to include your 2023 certificate of rent paid to claim the renter's credit. 

How do I amend my state tax return?

To add the certificate of rent:

1. In your MN state return continue to the screen Other Minnesota Credits
2. Select Renter's Credit 
3. Confirm that you meet all requirements and continue.