Not able to get "status" : "200 OK" for Bulk Load API Call using aws neptune

The curl utility does not have a means to pass IAM credentials. The error message you are getting is because you're using a Neptune cluster with IAM Authentication enabled. To resolve this issue, you can use awscurl [1], a utility that will sign the request with IAM credentials that you pass either directly as parameters, credentials that you have stored as environment variables, or credentials that you have stored as a profile in the AWS CLI.

awscurl -X POST --service neptune-db -H 'Content-Type: application/json' --region us-east-2 \
https://**.cluster-c4brigvg3m9m.us-east-2.neptune.amazonaws.com:8182/loader -d' 
    {
        "source" : "s3://*/Unsaved/2022/12/13/4a873928-9910-47b0-85ca-de593ace4f4a.csv", 
        "format" : "csv", 
        "iamRoleArn" : "arn:aws:iam::959061167427:role/NeptuneLoadFroms3", 
        "region" : "us-east-2", 
        "failOnError" : "FALSE"
}'

[1] https://github.com/okigan/awscurl