A manual relationship is defined with no columns.

How to fix

Add the missing sourceColumns and targetColumns arrays to your manual relationship.

Problem example:

{
  "manualRelationships": [
    {
      "sourceTable": { "schema": "dbo", "name": "Orders" },
      "targetTable": { "schema": "dbo", "name": "Customers" }
      // Missing sourceColumns and targetColumns
    }
  ]
}

Fixed:

{
  "manualRelationships": [
    {
      "sourceTable": { "schema": "dbo", "name": "Orders" },
      "sourceColumns": ["CustomerId"],           // Added
      "targetTable": { "schema": "dbo", "name": "Customers" },
      "targetColumns": ["Id"]                    // Added
    }
  ]
}