I spent many hours trying to find some way to create a new .NET Core 3.1 web app under Windows subscription. I found that if you pick up the Runtime stack as .Net Core 3.1 (LTS) the only option is to create an app under the Linux. I tried to play with different regions and Sku and sizes as well but for all cases, it's just the same. There is no option to configure App Insight during the creation time either when using the Linux. 
The interesting thing is that when I created the Web App under .net core 3.0 marked as current I was able to deploy .net core 3.1 web app and it all worked including the app insights stuff.
My question: is it just a bug in the Azure UI that you cannot create .net core 3.1 web app under Windows or there is some specific reason behind it?
2 Answers
This option is going to be available once the deployment of .NET Core 3.1 has been rolled out to all AppServices worldwide. So far, this is only been completed for Linux.
https://github.com/Azure/app-service-announcements/issues/217
https://github.com/Azure/app-service-announcements-discussions/issues/129#issuecomment-565620039
Windows hosted App Service option will be enabled once world wide rollout of 3.1 SDK to App Service backend is complete.
Since Windows AppServices have all required runtimes installed, you can select 3.0 and try to deploy your app regardles of what the portal says. I was able to host a WebApi in west Europe this week with .NET Core 3.1, but from what I read that might just have been luck. Deploying a self contained version is probably also possible from what I read in the linked Github issue.
edit2:Note that the accouncement talks about the SDK. As of the writing of this post, most of the app services have the 3.1 runtime installed, while the SDK will take a while longer.
edit: You can also check manually if your AppService has the runtimes installed already:
- Go to Kudu (https://myappservicename.scm.azurewebsites.net/)
- Open the Debug console (e.g. CMD)
- type in
dotnet --list-runtimes
I had the same problem. I could not get windows selected in Azure.
What I did was let Visual Studio create the App in my App Services. (Note the app insights!)
When this profile finished creating I went into the created App in the Azure portal. Settings - Configuration and changed the stack from 4.7 to .Net Core (Sorry this is in dutch, I hope you can manage...)
And now I also had the standard documents available which I did not have creating it in Azure portal! I changed it to only have index as landing page :
After I did this the homepage of my website showed up. Hope this helps someone!
