Windows Server 2016 Datacenter VMs running on Hyper-V Server 2016

I misread something or I'm doing a terrible mistake here, hopefully you guys can help me.

I have set up a Hyper-V Server 2016 on my physical machine (or bare metal as it's called) which has 1 processor with 4 cores.

Afterwards, I connected with Hyper-V Manager and I set up 2 VMs and installed Windows Server 2016 Datacenter in both of them. So far so good.

However, when I set up a 3rd VM and installed Windows Server 2016 Datacenter in it, Windows said that my key is already used in another machine.

But... I thought I can create unlimited VMs with the Datacenter edition... right?

What am I doing wrong?

EDIT1: I exported my VMs (the ones with the datacenter key) AND I removed hyper-v server from my host machine and erased it. Then I installed Windows Server 2016 Datacentre with the Hyper-V role and used the Datacenter key to activate it. Windows still reports that my key has already been used on a different device. Error code: 0xC004C008

EDIT2: I re-imported the VMs to my host machine and I removed the datacentre key from the VMs and put the AMVA key TMJ3Y-NTRTM-FJYXT-T22BY-CWG3J (I found it here) to all of them. The host machine though remains dis-activated

EDIT3: I called Microsoft and activated the host machine by phone. At first the automatic activation system told me that I cannot activate this key and that I had to get another one. But actually I had to intentionally make a mistake during the automatic phone activation in order to reach another human being on the line. Finally, after I spoke with a human, my host machine and VMs were activated without problems. Thank you all!

What you did wrong is adding the key. Which you needed because... ah.

• You should have installed Datacenter on Hyper-V. With a proper key that would auto activate any VM on it. Hyper-V on the host is not beneficial licensing wise in most cases, and technically means no auto activation.
• There is pretty much zero benefit from installing Datacenter on a VM. Benefits are mostly licensing, on a level that is not useful outside of the host (due to the host on Datacenter auto activating client VMs).

That sort of sums it up.