A dataset is defined as an expression dataset but no expression is defined.

How to fix

Ensure that the dataset has a valid expression or remove the dataset.

Problem example:

{
  "tables": [
    {
      "schema": "Person",
      "name": "Address",
      "columns": [
        {
          "name": "Email",
          "dataset": "EmailAddresses",
          "deterministic": true
        }
      ],
      "datasets": [
        {
          "name": "EmailAddresses",
          "type": "Expression"
          // Missing "expression" property
        }
      ]
    }
  ]
}

Ensure that the dataset has a valid expression or remove the dataset:

Option 1 - Add expression:

{
  "tables": [
    {
      "schema": "Person",
      "name": "Address",
      "columns": [
        {
          "name": "Email",
          "dataset": "EmailAddresses",
          "deterministic": true
        }
      ],
      "datasets": [
        {
          "name": "EmailAddresses",
          "type": "Expression",
          "expression": "$(GivenNames($[GivenName])).$(FamilyNames($[FamilyName]))@$(Domains($[Id]))$(TopLevelDomains($[Id]))"
        }
      ]
    }
  ]
}

Option 2 - Remove the dataset:

{
  "tables": [
    {
      "schema": "Person",
      "name": "Address",
      "columns": [
        {
          "name": "Email",
          "dataset": "SomeOtherDataset"
        }
      ]
    }
  ]
}