Hello. I’m trying to figure out where to enter a certificate of rent paid for my father.

Which state? 

Generally, the renters credit question, IF the credit is part of the tax return, will be asked as you walk through the steps.  Be sure to walk through all the steps and answer all the questions in the state section. 

If you live in a state that does the credit separately then you would need to get on your state website to submit your certificate or information to get the renters credit.