A dataset is defined as using a file but no file is specified.

How to fix

Ensure that you have supplied a file for the dataset using the file attribute

Problem example:

{
  "datasets": [
    {
      "name": "CustomCities",
      "type": "File"
      // Missing "file" property
    }
  ]
}

Ensure that you have supplied a file for the dataset using the file attribute:

{
  "datasets": [
    {
      "name": "CustomCities",
      "type": "File",
      "file": "cities.txt"           // Added file reference
    }
  ]
}

cities.txt example:

Seattle
Portland
Vancouver
Spokane