Please use the following link to contact apple support => https://getsupport.apple.com/solutions
Then select App Store > Subscriptions & Purchases > Unable to purchase.
Click on "Continue" when you get to this page the scroll down and click on "Get more help"
Then tap on the Live chat to speak with an apple support.
Just provide them your First Name, Last Name and Apple ID Email Address.
In the Additional Details textfield, paste the below message to them.
"Hello Apple Support,
I recently updated my payment method, and I’m now receiving an error stating: “Your purchase could not be completed” whenever I try to make a purchase. I contacted my bank, and they informed me that Apple has placed a restriction on my Apple ID. Could you please help resolve this issue?"