In app purchases

It is very hard for anyone to offer a solution unless you give a detailed description of what is happening when you attempt to make the purchase. Show screenshots of any alert sheets that appear. (Cover any personal information in the screenshot.)

Late 2012 Mac mini, macOS 10.13.1;  Watch S3, watchOS 4.1; iPad Air 2 & iPhone 6+, iOS 11.2; TV 4th Gen, tvOS 11.1;  Airport Express, AirPods

Then that is what you need to do. It could be an issue with a previous purchase that needs to be cleared up or a problem with your payment method. iTunes Support should be able to tell you.

iTunes Support -

http://www.apple.com/support/itunes/