I can tell you how to fix it. Ready? Add 1$ to your account through the App Store. Click add money to your account. Only 1$ it will push the pending through.

Technically, the purchase is not pending, as soon as the payment gets cleared from your bank account the purchase is deemed complete and final. Nevertheless, you can always contact iTunes support through Report a Problem and get refund for your purchase. Some developers bar refunds if the app has been downloaded at least once, since you haven't done so you need not worry about this caveat.

The portal shows purchases you made in the last 90 days, for older purchases you'll need to contact iTunes Support directly.

Follow these steps.

1. Go to Apple's Report a Problem portal and sign in using the Apple ID used for purchase.

2. You should see a list of your recent purchases (last 90 days). Find the app in question and tap on report.

3. Describe your problem and click submit.

Apple Support will write to you with 3-5 business days. All the best.

Reference: Apple Support

Update your billing informationYou can update your billing information in Settings on your iPhone, iPad, or iPod touch, or in iTunes on your Mac or PC. When you update your information, use any of these payment methods. You can't select None when you have an unpaid balance or payment due.After you update your billing information, the App Store or iTunes Store automatically attempts to charge any outstanding orders to your updated payment method.Pay an unpaid billOn your iOS device: Go to Settings > [your name] > iTunes & App Store, then tap your Apple ID. Tap View Apple ID and sign in with your Apple ID, then tap Purchase History. (You can also view your purchase history in iTunes on a Mac or PC.)Find the unpaid purchase, which is likely at the top of the list, and tap it, then change your payment method.Add funds to your Apple IDWhen you try to pay an unpaid bill, you might see the option to pay with Apple ID credit if you have a credit balance.If you don’t have a credit balance, you can redeem App Store & iTunes Gift Cards or add funds to your Apple ID2, then use the credit to pay an unpaid bill.