Not able to use API credits

I’ve received credits of worth $500 but still cant use them. These credits were offered by OpenAI for research purpose but while making the API requests, it throws error as

“error”: {
“message”: “You exceeded your current quota, please check your plan and billing details. For more information on this error, read the docs:”,
“type”: “insufficient_quota”,
“param”: null,
“code”: “insufficient_quota”

looking at your usage page, what happens if you increase your limit to $500?

Since i am on free plan, cant change. (These Credits are provided as research grant )

It gives warning as: “You must be on a paid plan to manage usage limits”