It sounds like you have Ask To Buy activated on your account:
Approve what kids buy with Ask to Buy - Apple Support
If your parent (I have no idea of your age) has done this then they will have to either approve your purchase or deactivate the feature.
If nobody has activated it then try signing out of your account and back in again and/or restarting your device. If that doesn't work then:.
Click here --> Choose your country or region - Official Apple Support Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.
or:
"See a list of Apple phone numbers around the world."
Click here --> Contact Apple for support and service - Apple Support