Trying to purchase an app and it won’t purchase

Hello,

See these steps:

• Open the App Store.
• Click your name. If your name doesn't appear, click the Sign In button, sign in with your Apple ID, then click your name.
• Click Account Settings.
• Next to Payment Information, click Manage Payments. ... 
• Click Add Payment.
• Enter the payment method details, then click Done.

Also see:

Add a payment method to your Apple ID - Apple Support

Good luck.