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 upstream charge over and above the subscription charge. You can read more about Paid API’s hereWithout sufficient credits, the API wont work. 

You can learn about which API’s have an upstream cost by visiting their guide.

Note: Absence of a tick mark indicates that there won't be any upstream cost 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 upstream cost 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!