How to claim Au pair upfront fees paid in installments.

I have a question in regards to claiming the Au Pair large agency fee which is eligible for Child care Tax credit. 

I paid it in installments with most payments made in 2021 and some payments - in 2022. 

Should I claim on 2021 Tax return only payments made in 2021 or can I claim the whole fee in the 2021 Tax return?

Thank you.