Python version error while deploying Flask app on vercel

Question

I am trying to deploy a Flask app on vercel

vercel.json

{
    "version": 2,
    "builds": [
        {
            "src": "./index.py",
            "use": "@vercel/python",
            "config": {
                "runtime": "python3.9"
            }
        }
    ],
    "routes": [
        {
            "src": "/(.*)",
            "dest": "/"
        }
    ]
}

And I am getting this kind of error

Error: Unable to find any supported Python versions.

After reading the documentation I found that it requires Python 3.9 , updated the same , but still the error remains the same!

Help appreciated!