It looks like your question hasn't gotten an answer yet. Providing the following information might help others in the Apple Support Communities help you get a solution:
Where are you trying to make purchases from, the iTunes or App Store directly, or from within an already purchased app? Do you recieve an error message when you try to make purchases? If so, what is the exact message you receive? What is the current iOS version installed on your iPhone 8 Plus (from Settings > General > About > Version)?