Clarence wrote:

Please remove quotation mark from query.
Command should like this:
https://simplex.sgvmart.com/odata/product(635b6d4c246dad138307b717)

It's still 404, see the link for attached image

It's still 404, see the image to the screen capture.
https://file.simplex.sg/s/efNmgBS6CcMrezJ

I'm sorry I didn't check before. Every methods you find in the swagger https://demo.grandnode.com/swagger/index.html I've check delete and it works correctly

I'm at my wits end. It is strange that I can create product using the API and encounter 404 for product delete.
How can I install the plugin in the development version i.e. v2.21 to try out?

To delete you should use this url:
https://store/odata/product?key=6356b046a85d437deaedb6de
You have everything describe in the open api format.
It available here: https://demo.grandnode.com/swagger/v1/swagger.json
or
https://yourstore/swagger/v1/swagger.json

I've tried https://simplex.sgvmart.com/swagger/v1/swagger.json and still get 404 error.
Using Postman DELETE https://simplex.sgvmart.com/odata/product(6356b046a85d437deaedb6de) also get 404 error.

So far only POST https://simplex.sgvmart.com/odata/product and POST https://simplex.sgvmart.com/odata/picture work.
Others resulted in 404 errors.

Could it be GrandNode2 v2.0 installation having issue? I would like to try installing GrandNode2 v2.1. How do I install the Extended Web API plugin on the development version?

I've tested on stable version 2.0.0
Develop version is not stable and any plugins will not be work with this version. After official publish version 2.1 the plugins will be updated.
Please find the file appsettings.json
and change setttings:
"BackendAPI": {
    "Enabled": true,
    "UseSwagger": true,
..