Is your Shopify API Private Key for your app in production correct? If it’s using your development app’s private key, then this would fail authentication.
Are you certain this merchant’s API key is correct? One potential issue is if the merchant uninstalled and then later reintalled, but your app kept their old token and didn’t overwrite it with their new one.
If all API requests are failing, I suspect #1 is your issue. If it’s only this merchant and all of their Shopify Admin API requests are failing then perhaps you have the wrong access token stored on their account, or perhaps your app is looking up the wrong token.
Actually its a live app and there are other merchants who are able to use the app without any issues. only this merchant faced an issue and they uninstalled the app. So I am not sure if their API key was correct was not.
In Gadget, they maintain a database against the webhooks you subscribe to and sync it regularly as per a schedule you set. In the above context, the Store data was getting synced for the 1st time for registered webhooks like product etc