You do need to have money in your account if you’re attempting to make a purchase and not simply downloading a free app.
It’s possible there’s a pending charge that’s leading to issues with your payment method. You can check your purchase history in the iTunes and App Store by following these steps: https://support.apple.com/en-us/HT204088
You can also follow these steps to help determine why your payment method is being declined: https://support.apple.com/en-us/HT203005
If all else fails, I recommend contacting Apple Support via one of the following methods:
• Call 1.800.275.2273
• Visit https://getsupport.apple.com
• Download the Apple Support app https://itunes.apple.com/us/app/apple-support/id1130498044?mt=8