Hello,
Below are my answers regarding your questions:
Can you try clearing cookies and cache for your browser and see if issue persists? Can you also try using a different browser?
A- Yes, I've tried with Google Chrome and Firefox without success. The same issue happen from InPrivate Windows option from any browser
Can you try deploying the application gateway resource using Azure PowerShell or using Azure CLI ? and use the existing Virtual Networks, please let me know if you get any errors which might give us more insights. If the error is regarding permission required can you check your Azure Role Assignment as mentioned in the next step. Also please make sure that these virtual networks have a dedicated subnet for Application Gateway. Please go through this documentation for Virtual network requirements and Subnet Size.
A- From PowerShell I'm not face the issue. I'm using an account with the highest Azure Role. The dedicated subnet for Application Gateway already exist.
Can you check what Azure role assignment you have for your subscription or the resource groups listed above? I think you at least need to have Virtual Machine Contributor role in order to add Application Gateway/ Azure Load balancer backend pool. You can also explore some additional roles and see any other roles suits your requirements. Please let me know if you any additional concerns here.
A- I'm using an account with the highest Azure Role
Waiting for your reply soon.
Best Regards