It looks like Windows Admin Center, needs to be deployed directly from Azure for the Arc server.
- Open the Azure portal and navigate to your Arc-enabled server.
- Under the Settings group, select Windows Admin Center.
- Specify the port on which you wish to install Windows Admin Center, and then select Install
If you don't get this prompt, are you able to supply a screenshot so we can dig a bit further?
Reference: https://azureblog.org/rdp-for-on-prem-vms-with-azure-arc/