In-app purchases (IAPs) are mediated by Google or Apple, depending on the device you use. Generally, all IAPs can only be completed with a credit card, and payment methods accepted are defined by the respective app stores. Subscriptions, for example, can only be purchased if you have a credit card associated with your account. This is because subscriptions will automatically renew at the end of each period and you will be charged for the renewal.

If you already have a credit card in your Apple/Google account and you're having problems to complete a purchase, here are some troubleshooting steps that usually help:

- Check that you have a WiFi or mobile data connection which is active and working. A simple way to do this is by searching the web. If you see information on the search you made, your internet connection is probably not the issue. If that search doesn’t work, connectivity might be a problem.
- Force stop, then reopen the game and try to make the purchase again.
- Check if the payment went through. If your payment didn’t go through properly you won’t receive the in-app purchase you tried to buy. Check if it went through by verifying your purchase history inside the app store.
- Restart your device. Sometimes restarting the device can help fix in-app purchase issues.
- Make sure date and time are updated. If you have recently changed the date and time on your device, make sure the date and time are correct.

Also, here are a few common issues related to the app stores, according to your device's platform:

Apple (iOS):

If you have an unpaid balance in the App Store or iTunes Store, you might see one of these messages: 
"There is a billing problem with a previous purchase. Please edit your billing info to correct the problem."
"There is a billing problem with a previous purchase. Click Purchase History to view and correct the problem. If you cancel you will not be able to buy until this billing issue has been resolved."

If your financial institution declines a charge or there's an issue with your payment method, your order will become unpaid. You won’t be able to make other purchases or update apps, including free apps, until you update your billing information to pay any unpaid bills.

- Find an unpaid bill.
1. On your iOS device: Go to Settings - [your name] - iTunes & App Store, then tap your Apple ID.
2. Tap View Apple ID and sign in with your Apple ID, then tap Purchase History. (You can also view your purchase history in the App Store or iTunes Store.)
3. Find the unpaid purchase, which is likely at the top of the list, and tap it, then change your payment method.
4. You must pay for any unpaid purchases or subscriptions before requesting a refund.

- Use a different payment method.
You can change your billing information in Settings on your iPhone, iPad, or iPod touch, or in iTunes on your Mac or PC. Use a valid payment method in your country or region with money available on it. After you update your billing information, the App Store or iTunes Store automatically attempts to charge any outstanding orders to your updated payment method.

If you still need help, contact Apple Support.

Google (Android):

- Make sure your card information is up to date in Google Payments.
Expired credit cards or old billing addresses are a common reason for payments not to work properly.

- Try a different payment method.
If you're trying to make an in-app purchase but the transaction is declined or the payment won't go through, try to change your payment method. Sometimes a different payment method will help your transaction go through.

To change your payment method:
1. On your Android phone or tablet, try to make a purchase in the app of your choice;
2. When the pop-up with the price shows up, click on the arrow next to the payment method;
3. Choose a different payment method or add a new one.
4. Complete your purchase.

- Remove or update expired cards.
Expired cards cause payments to be declined. To update expired cards:
1. Sign in to with your Google Account.
2. Find the payment method you are trying to use for the purchase.
3. Check the expiration date of the payment methods listed.
4. Remove or update any expired payment methods.

- Check if your zip code is up to date.
If your credit card is registered to a different address that can cause the payment to be declined.

Check the zip code matches your current address:
1. Sign in to with your Google Account.
2. Find the payment method you are trying to use for the purchase.
3. Click Edit.
4. Check that the zip code listed matches the zip code of the billing address of your card.
5. If needed, update the zip code.

- Force Google Play to stop, and clear the cache and data.
First, Force the Google Play to stop:
1. On your Android phone or tablet, open your device's Settings app.
2. Scroll down and tap Apps or Application manager (depending on your device, this may be different).
3. Tap Google Play Store.
4. Tap Force stop.

Second, clear the cache and data for Google Play:
5. On your Android phone or tablet, open your device's Settings app.
6. Tap Apps or Application manager.
7. Tap Google Play Store.
8. Tap Storage.
9. Tap Clear cache.
10. Tap Clear data.

Next, reopen the Google Play:
11. On your Android phone or tablet, open the Google Play Store app.
12. Try your purchase again.

If you still need help, contact Google Support.