My NDVI image is coming 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 i.e. 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, and the sample code is based on analytic and basic_analytic asset types of the following item_types:

• PSOrthoTile
• REOrthoTile
• PSScene4Band

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.