Zynga Poker Purchase with Apple Pay

Where exactly is this balance you say you have?

Do note, that Apple Pay Cash, and your Apple ID balance are 2 different things.

Apple Pay is not directly used to pay for things from Apps or in the App Store.

The balance its referring to in that message is your Apple ID (gift card credit) balance.

See here: link-> Check your gift card balance - Apple Support

It says it's "insufficient". Meaning there may be some, but not enough to cover the purchase.

How much balance do you have, and how much does the item you want to buy cost?

Remember, that in many countries the prices in the App Store are before taxes. So you need to add whatever sales tax is applied in your location to get the final price that your balance needs to cover.