"Your Windows license will expire soon"

Go back to the shop demand a genuine RETAIL key,
Reporting him to MS >might< get you a key.

^^^^^ This ^^^^^

Reselling a PC that is Volume Licensed is illegal, period, no ifs ands or buts. He may have gotten it with Windows Preinstalled, but that still doesn't make it legal to resell it with that particular version. Legally activated Home or Pro yes, Volume License no. The Volume License is owned by the company and stays with that company, it's not transferable. It could be that he got the PC second hand. I've seen it before, the IT guy gets lazy and doesn't wipe the company install from the PC like they are supposed to. Or the company just went belly up and the assets are sold as is. Then we end up where we are now. Activation times out when that PC can no longer connect to the company KMS server. That's quit often only possible if you have access to that companies internal network.

The other scenario is the guy in the PC shop just blatantly pirated Windows by setting up or connecting to a fake KMS server. That happens to, all to frequently IMHO.

Either way, this PC will fail to activate and you need to get a valid Product code/Licensee to keep using it.

The -3V66T key is the generic key Pro gets with a Digital License. It could be that you did have a valid Pro DL, then something messed up activation, the update maybe, hard to say. What you did to try and fix it may be why you now have the KMS key. You haven't mentioned what you did. I'm not passing judgement by the way. You wouldn't be the first one to try something posted on the internet only to find out latter, that's not the way you do it, legally. Or you made a typo that had unexpected results, it happens.

What you could do is do the "change product key" option on the activation screen and type in the full Pro key showkey found. The key should be VK7JG-NPHTM-C97JM-9MPGT-3V66T. That key is well know and easily found with a Google search. It only works if that PC already has a DL stored on the activation server. Everybody that did the free upgrade to Pro got that generic key. I get it if I activate my pro installs with a skip key. They already have a DL on the activation server. If not the PC won't activate. It can't be used to get Pro for free. If your PC was originally activated with a Pro DL, entering that key should fix it. If it doesn't have a DL it won't activate and you'll get an error message or reason why. Even if that happens, IMHO, your in no worse a spot than you are now. There is no legal way to activate your Pro install with a KMS key. Not without buying a license for one from Microsoft. Way too expensive even if they would sell you one, which they won't. Not a single anyway.

Try the troubleshoot option at the bottom, and or open an admin command prompt and run slmgr /ato.

NOOO! Did you save the installed key somewhere? That one is the one you need...it is a retail key.

I've gotten activation error messages, even though I have a valid digital License. I can't remember if I had the exact same message shown above though. I should have taken a screen shot at the time but didn't.
Running slmgr /ato usually fixes it for me.