Domain Join Error while deploying HostPool

GR_C1pD 

WVD requires both Azure AD (AAD) and Windows Active Directory (AD).  This means you will need to either deploy the Domain Controller role on a VM running in Azure, or use an existing DC on-prem.  Since you already have a DC on-prem with AAD connect, it would be ideal if you could connect your Azure VNET and your on-prem network together (S2S VPN, Express Route), that way, VM's in Azure could join your Windows AD domain on-prem.  If you had the network setup between them, you could also extend Windows AD into Azure by creating a VM in Azure and adding another DC to the domain and that is the best recommendation.

Since you already have Windows AD and AAD connect on-prem, you dont need AADDS.  AADDS is great for those who don't have any DC's running anywhere and are not familiar with setting up Windows AD because the VM's, the DC role and domain are deployed and managed for you as an Azure service.

Also, if you try setting up a new DC in Azure and create a new Windows AD domain and then try to sync to an existing AAD with AAD Connect, it is not supported.  Different Windows AD forests must be synced through a single AAD connect sync server.

If you just want to test WVD in a lab environment and have zero impact with your on-prem environment, I would create a new Azure AD domain, then create a VM in Azure to become a DC for a new Windows AD domain, then deploy AAD connect and sync.  From there, you should be able to manually domain join a VM to that Windows AD domain and if so, you should be able to proceed with WVD host pool creation.

-Jeff