Error using REST API Basic Authentication in Azure ADF

Error: Input is malformed. Reason: Could not get entity details for DandB Test. If this entity is a linked service, please ensure it has been published before using in debug runs.

As per the error you are providing impropriate inputs or ADF pipeline is not getting the entities you are providing pipeline parameters and check the expression you are using is in correct format.

Some users also solved the issue by storing the password in key vault secret and then accessing it in ADF you can also try it.

• First save your password in Azure Key vault as secret.
• Create linked service for it.
• Then select Azure key vault checkbox for password and select your key vault.
• Add your keyvalt linked service and select your secret.

And try to debug the pipeline

I tried your pipeline in my environment its working for me without this Input is malformed error.

Mypipeline.json

{
    "name": "DandB",
    "properties": {
        "activities": [
            {
                "name": "Web1",
                "type": "WebActivity",
                "dependsOn": [],
                "policy": {
                    "timeout": "0.12:00:00",
                    "retry": 0,
                    "retryIntervalInSeconds": 30,
                    "secureOutput": false,
                    "secureInput": false
                },
                "userProperties": [],
                "typeProperties": {
                    "method": "POST",
                    "headers": {
                        "Content-Type": "application/x-www-form-urlencoded",
                        "Cache-Control": "no-cache"
                    },
                    "url": "https://plus.dnb.com/v3/token",
                    "body": {
                        "value": "@concat('grant_type=client_credentials&client_id=',pipeline().parameters.ClientID,'&client_secret=',pipeline().parameters.ClientSecret)",
                        "type": "Expression"
                    },
                    "authentication": {
                        "type": "Basic",
                        "username": {
                            "value": "@pipeline().parameters.client_id",
                            "type": "Expression"
                        },
                        "password": {
                            "type": "SecureString",
                            "value": "**********"
                        }
                    }
                }
            }
        ],
        "parameters": {
            "ClientID": {
                "type": "String"
            },
            "ClientSecret": {
                "type": "String"
            },
            "client_id": {
                "type": "String"
            }
        },
        "annotations": []
    }
}

From output section you can check you are passing correct input: