I have an app in azure and wanna to enable "mysql in app" feature inside it to use MySQL. I read many articles but they say we must enable it by clicking on "mysql in app" menu and toggle on off button.
But as you see in below picture this option is deactivated in my portal?
What is my mistake?? How can I activate it?
It looks like you are using a Linux-based Web App, MySQL in-app is not available for Linux App Service plans.
You could either create a Windows based Web App or use a Docker container to host both, your application and a MySQL database on Web Apps (see also here).