Insufficient balance

It may mean you don't have enough Apple ID balance to pay for something and you don't have a payment method set as a backup.

Read the links below about various ways to pay for items in the iTunes and App Stores.

- Add money to your Apple ID balance - "With the payment method that you have on file, you can add funds to the Apple ID that you use for iTunes & App Store and iCloud Storage purchases."

https://support.apple.com/HT208687

- Payment methods that you can use with your Apple ID - https://support.apple.com/HT202631

then

Change, add, or remove Apple ID payment methods - https://support.apple.com/HT201266

- Create and add money to an iTunes Pass - "At Apple Stores and select retailers, you can add store credit directly to the Apple ID that you use for App Store, iTunes Store, and iBooks Store purchases." - https://support.apple.com/HT203021

- Redeem App Store & iTunes Gift Cards, Apple Music Gift Cards, and content codes - https://support.apple.com/HT201209