When I try to download an app that needs payment, when I put in my card details it comes up with this form has not been complete

Hi Sunnyc3ntral,

A good first step is to ensure that the payment method you're attempting to add is supported in your region. The article Payment methods that you can use with your Apple ID outlines by country acceptable methods you can use.

Next, review the options at Add a payment method to your Apple ID and consider using a different method to add your payment method such as at the Apple ID website. Please also note that the billing address for the payment method must match the country or region that your Apple ID is presently configured for.

Should those steps not help, If your payment method is declined in the App Store or iTunes Store offers the following:

"If you still can't make a purchase

• You might have an unpaid order. Learn how to pay an unpaid order.
• Contact your financial institution. 
  • You might need to ask your financial institution to allow international purchases on your debit or credit card.
  • If you want to know why a debit card, credit card, or other payment method was declined, speak with your financial institution. Apple doesn't have information about why the payment method was declined.
• In some countries and regions, your payment method might be declined because of changes to local law. Learn about billing and purchases in Europe or what to do if your card is declined in India.
• Still need help? Contact Apple Support."

Regards.