Hello @kishor,
I can understand that you're trying to create an Airtable extension block.
Follow these instructions https://airtable.com/developers/extensions/guides/hello-world-tutorial

This is not run on our local browser but it's run on the Airtable extension panel where we added that custom extension.

I hope this helps you.
👍

i did done those steps, even on the documentation itself says that its runs on the local host cause its simply a react app
well its also points index.js in front end like this
{
2 │ "version": "1.0",
3 │ "frontendEntry": "./frontend/index.js"
4 │ }

but i dont actually get this point that you mentioned 
"   This is not run on our local browser but it's run on the Airtable extension panel where we added that custom extension.   "

i will be greatfull if you can provide more information 
thank you

@kishor 
When we run https://localhost:9000 to our browser on a single tab then It's run independently on our local browser.

However, these custom extensions are not independent. They are dependent on the airtable.com domain.
So it needs to run within Airtable to use predefined UI block, use loaded data and some settings that are within the Airtable domain.

I hope you can understand now.

so that means even the "Hello World" works with in the local enviornment right ?
 

Use the Local environment for development; start run from the local system but output is always shown on Airtable.

Please read this carefully again.  https://airtable.com/developers/extensions/guides/hello-world-tutorial.

update:
Fixed it by changing the browser to firefox , 
i have no clue why its not working in chrome,brave,edge , may be some sort of config to connect to localhost from browser has some issues