Hello tu90,
It sounds as though you're being prompted to verify your Apple ID when attempting to download an application. Apple Support Communities is a great place for help. To clarify a bit further, are you being prompted to verify your payment details? If so, you should be prompted to re-enter the verification code on your card. If you're entering the code, and it's not being accepted you may need to reach out to Apple for additional support. You can find their contact details via the Get Support website.
If the payment method is being declined the following article outlines the steps to take for reaching a resolution.
If your payment method is declined in the App Store or iTunes Store
If you're having a different issue such as continuously being prompted to re-enter your Apple ID password try restarting your iPhone as outlined in the following article.
Restart your iPhone
We look forward to the results, have a great day!