REST API to add data into a data extension leads to error 20002

I am using a REST API to add data into a data extension and I receive the following error:

{
"message": "Insufficient privileges to complete this action.",
"errorcode": 20002,
"documentation": ""
}

This API was configured around 2 months ago and it was working fine that time. In between I am not sure if I made some changes unknowingly.
Please help me to resolve this error at the earliest.