Hi, my name is Neil, I'm an independent adviser and a Windows 10 user like you.
You are correct that this is definitely a missing file of DirectX.
I think installing DirectX 11 would fix your issue for you since this was the DirectX version recommended for Civ 6 when it came out.
Often, the game itself usually includes a copy of the DirectX installer. Try looking at the game installation folder and see if you can see a folder or file called DirectX. If so, click on it and install it.
You can install DirectX 11 even though you have DirectX 12 already installed. It will just place the missing files needed for your game. It won't actually replace your current DirectX 12 with DirectX 11.
Unfortunately, every Microsoft link I can find to download DirectX 11 seems to have been removed from their pages :/
However, there are lots of websites that you can still find the DirectX11 installer on. I cannot recommend any to you as I simply haven't used any before, but please, if you have to download from somewhere, make sure it is a safe website that is well trusted and there are people who have downloaded it from there and can attest to its authenticity.
I hope this helps!