If we paid all adoption fees in 2017 and the adoption wasn't finalized until Feb 2018, do we have to wait until 2018 to file for the adoption credit?

Yes. Per the IRS, you would not be able to claim the adoption expenses until you file your 2018 return.

If the child is born in the United States or is a resident alien, then you may claim the adoption tax credit in the year after you incurred the expenses.

Please feel free to post any additional details or questions in the comment section.