A manual relationship is defined with a differing number of source and target columns.

How to fix 

Ensure the number of source and target columns match in your manual relationship.

Problem example:

{
  "sourceColumns": ["CustomerId", "ProductId"],  // 2 columns
  "targetColumns": ["Id"]                        // 1 column - MISMATCH
}

Fixed:

{
  "sourceColumns": ["CustomerId"],  // 1 column
  "targetColumns": ["Id"]           // 1 column - MATCHES
}