Hi @Miguel Angel Corrales Serrano ,
I did come across the same experience as you have. I will need to confirm with the product group but currently, App Service integration is in preview, I would say traditional app services isn't yet available. In my case, I could not even select an APIM instance. As soon as I get more information, I'll update my post.
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EDIT 2022 September 16 I am able to see app services from the drop down menu when Web App is selected. Please try the operation again and if you run into the same issue, please a capture network trace from the browser.