Please check if below cases :
Note :
- Limitation : App Service on Linux is not supported on Shared pricing tier.
- Linux Azure Web Apps uses pm2 to serve a node app
Go to Azure app service> Configuration > General Settings.
If your build folder is at the root of the project,give Start up command: pm2 serve /home/site/wwwroot --no-daemon –spa
Please go through these related issue References to trouble shoot :
- reactjs - Web App Linux - Stack Overflow
- React App not starting in azure app service - Stack Overflow
Also please check this Blog.
According to Azure App Service | Microsoft Docs
This error could be due to one of several reasons:
The custom domain configured is missing an A record or a CNAME
record.
The web app owner has moved the web app to a different region, but
the DNS cache is still directing to the old IP Address that was used
in the previous region.
- Clear the browser cache and cookies for the site and test for DNS
resolution/ dns entries if they are still pointing to an old IP.Edit
and correct dns in that case.
In your app page, select TLS/SSL settings. Then make HTTPS Only to On.

Also, suggest you Enable diagnostics logging for web apps in Azure App Service incase if you haven’t enabled earlier to check the complete error details and root cause.