Ok, what you have there is not a DC joined to your AD forest, its a DC leveraging Azure Active Directory Domain Services
It essentially a managed domain and you can not install AADConnect there.
https://learn.microsoft.com/en-us/azure/active-directory-domain-services/overview
The way it would work is you install AADConnect on a domain server, it would sync to Azure, then Azure would sync to Azure Active Directory Domain Services
https://learn.microsoft.com/en-us/azure/active-directory-domain-services/synchronization