Hi Kattekola Mahesh,
Welcome to the Microsoft Q&A Platform! Thank you for reaching out regarding your query on Function app.
Based on the error message, it seems that the connection string ESG-IOT_events_IOTHUB is either missing or incorrectly configured in the App Settings of your Function App. Additionally, ensure that your function code is referencing the correct key for accessing the connection string. Before making any updates, please verify that the Event Hub's shared access policy has the appropriate permissions.
Let us know if you need further assistance!