Thanks for reaching here! Could you please share the content of the JSON tab so we can identify more details on the error message/code.
You may want to know that the validation errors arise from scenarios that can be determined before deployment.
They include syntax errors in your template or trying to deploy resources that would exceed your subscription quotas.
Also note that validation errors don't appear in your deployment history because the deployment never started. So, when you receive the error message, click on the error message to fetch error and error code details.
Follow the ‘recommendation’ in the error message to rectify the problem.
Further adding details on to buy an App Service domain, follow these steps.
- In the Azure portal, select the subscription to use to buy the domain.
- Select the resource group to put the domain in.
- Type the domain you want. For example, contoso.com. If the domain you want isn't available, you can select from a list of suggestions of available domains, or try a different domain.
- Select Next: Contact information and supply your information as required by ICANN for the domain registration. It's important that you fill out all required fields with as much accuracy as possible. Incorrect data for contact information can result in failure to buy the domain.
- Select Next: Hostname assignment and verify the default hostnames to map to your app.
- Select Next: Advanced and configure the optional settings.
App Service domains, exclusively managed within Azure, allow for seamless administration of custom domains for Azure App Service.
Suggest you refer this guide will walk you through the process of purchasing an App Service domain and configuring it for use with an App Service app.
: https://learn.microsoft.com/en-us/azure/app-service/manage-custom-dns-buy-domain
Let us know if further query or issue remains.