You have to first register your provider: "Microsoft.Web" for the relevant subscription. Go to the Resource Provider under Subscription Settings and type "Microsoft.Web" and click on "register".

Some general more involved steps:

1. On the Azure portal menu, search for Subscriptions. Select it from the available options.
2. Select the subscription you want to view.
3. On the left menu, under Settings, select Resource providers.
4. Find the resource provider you want to register.
5. Select the resource provider to see the details of the resource provider.
6. Select the resource provider, and select Register.

Reference Link: Resource providers and types