My NDVI Image is 0 or Full Black

It can be due to incorrect "pixel_type". If your "pixel_type" is an integer you will not be getting variation (-1 to 1), you will be getting only two values, 0 or 1. In a lot of cases, it will be only 0 (due to rounding of floating values to integer values).

Planet API allows users to do on the fly NDVI Calculation. This requires users to have a valid Planet API key. The sample code is based on analytic and basic_analytic asset types of the following item_types:

• PSOrthoTile
• REOrthoTile
• PSScene4Band
• PSScene8Band

You can use this sample code to try the NDVI calculation on PlanetScope 4 Band image.

{
   “name”:“ndvi_bandmath”,
   “products”:[
      {
         “item_ids”:[
            “20210306_042752_52_2276”
         ],
         “item_type”:“PSScene4Band”,
         “product_bundle”:“analytic”
      }
  ],
  “tools”: [
    {
      “bandmath”: {
        “b1": “b1”,
        “b2": “b2”,
        “b3": “b3”,
        “b4": “b4”,
        “b5": “(b4-b3)/(b4+b3)“,
        “pixel_type”: “32R”      }
    }
  ],
   “delivery”:{
      “archive_type”:“zip”,
      “single_archive”: true,
      “archive_filename”:“{{name}}_{{order_id}}.zip”
   }
}

The output of this tool will be an asset that includes all four bands of the original file and the fifth band with NDVI values.

Note: You can change the item_ids to desired Planet Imagery. The "32R" signifies a 32bit floating-point data type.