I did a blog article on this one:
vSphere ESXi 6.5 – Unable to take quiesced snapshot | The time I've wasted on technology...
I usually see 2 things cause this, but this is the most usual culprit.
Log on to the 2012 VM, and delete the volume shadow copies. Restart the VM (may not be needed), and then try taking the snapshot.
I see this occur when the VM has TONS of volume shadow copies. I regularly delete a bunch of mine to keep things running fast and smooth (and to allow the snapshots to work).