A column has been defined without a dataset.

In order to mask the data contained in a column, the dataset to obtain the new values from needs to be defined.

How to fix

Problem example:

{
  "tables": [
    {
      "schema": "dbo",
      "name": "Users",
      "columns": [
        {
          "name": "FirstName",
          // Missing "dataset" property
        }
      ]
    }
  ]
}

Add a dataset property to specify where replacement values come from:

{
  "tables": [
    {
      "schema": "dbo",
      "name": "Users",
      "columns": [
        {
          "name": "FirstName",
          "dataset": "FirstNames"        // Added dataset
        }
      ]
    }
  ]
}