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An insufficient Credit Error occurs when you use a Paid API without having sufficient credits in your Sandbox Wallet. The Error would look like this:
Paid APIs have an additional wallet charge over and above the subscription charge. You can read more about Paid APIs here. Without sufficient credits, the API won't work.
You can learn about which APIs have a wallet charge by visiting their guide.
Note: The absence of a tick mark indicates that there won't be any wallet charge on using the API.
How to resolve it?
You can add a minimum of 100 INR to your sandbox wallet in order to overcome the error. The wallet charge will be deducted in a prepaid manner on a per-call basis. You can click here to add balance to your wallet.
And that's all! Hope that it Helped!