If this message pops up:

"Insufficient Remaining Store Credit. Tap Continue to sign in."
This means that your balance in the App Store is insufficient to complete the process; enter your password, then click on continue to log in and follow the instructions.

For example:
If your store balance is 5$ and you want to buy a package worth 5$, you will get this message because there are taxes imposed by Apple and the store's country on the pack's final price.