My guess would be that the variable `$option` you're creating doesn't follow the guidelines you find in the Magento DevDoc.

Payload should be

    {
      "option": {
        "attribute_id": "141",
        "label": "Size",
        "position": 0,
        "is_use_default": true,
        "values": [ // -> this is an array
          {
            "value_index": 9 // -> this is an object within an array
          }
        ]
      }
    }

What you're creating is:

    $option = [
        "option" => [
            "attribute_id"=> "63",
            "label"=> "Size",
            "values"=> [ // -> this is an object
                [
                  "value_index"=> 54164121323 //-> this is an object within an object
                ] 
            ]
        ],
    ];


What is the response you get from http://localhost/magentoTest5/rest/V1/configurable-products/747682908/options? 200?