You need to active licenses in your Windows Server environment.
There are two ways to activate Windows Server:
Either way what needs to be done is depending on the version of Windows Server running on the Hyper-V host:
- Windows Server Datacenter. If the hypervisor host is running Datacenter edition, then the KMS/MAK key must be used for the activation of the host running Datacenter edition. For all virtual machines running underneath the datacenter host, the Automatic Virtual Machine Activation (AVMA) technique should be used. Details about how to activate virtual machines by using the AVMA method can be found at: https://docs.microsoft.com/en-us/windows-server/get-started-19/vm-activation-19
- Windows Server Standard. If the hypervisor host is running Standard edition, then the AVMA method is not supported and therefore both the host and the virtual machines must be activated by using a KMS or MAK key.
If the hypervisor host is not Hyper-V then the only way to activate Windows Servers is via either KMS or MAK keys for each of the corresponding Windows OS running on each server.
Note that the license key can be obtained by any available Microsoft licensing channel, including Volume (SPLA) and Open licensing.