Unable to sign in to App Store says 'Please contact iTunes to complete this transaction'

To contact iTunes Support, do this: 



1. Click on this link --> Choose your country or region - Official Apple Support  

2. Select your country

3. Select: Billing & Subscriptions

4. Select: Subscriptions & Purchases

5. Select: Update payment method

6. Select a chat session or a phone call.

Copied from iTunes support - Apple Community