A column is defined that does not exist in the database.

How to fix

Remove the column from the masking configuration.

Problem example:

{
  "tables": [
    {
      "schema": "dbo",
      "name": "Users",
      "columns": [
        {
          "name": "FirstName",
          "dataset": "GivenNames"
        },
        {
          "name": "MiddleName",          // Column doesn't exist in database
          "dataset": "GivenNames"
        }
      ]
    }
  ]
}

Remove the missing column:

{
  "tables": [
    {
      "schema": "dbo",
      "name": "Users",
      "columns": [
        {
          "name": "FirstName",
          "dataset": "GivenNames"
        }
      ]
    }
  ]
}