I can’t purchase anything in App Store

First ensure that you can connect to the iTunes Store.

https://support.apple.com/HT201400

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Then try again and if the problem persists make sure your billing details are up to date

https://support.apple.com/en-us/HT203005

And if that doesn't work either go here https://support.apple.com/itunes and contact apple support.

It seems as though you have a popup message that says: Contact iTunes Support --> http://www.apple.com/support/itunes/ww

Please do so...

- Selecting your country

Watch the GIF below

What happened when you did as instructed and contacted iTunes Support at https://www.apple.com/support/itunes/ww/?