The Google Play Movies app purchase is "not supported on this device". Why?

You are making it more complicated than it is.

Google provides an ios app for viewing purchased movies and show.

They dont allow purchases because they don't want to give give Apple a cut of the money from the sale.

You have not provide any information about

Your status as Minor or Major

iPhone restrictions if any

Which country are you residing

However...

Google Play Movies & TV is not an Apple product. Contact the app developer here --> App Support 

That would be a question to ask the app developer.

Because Google doesn't allow it.