The brain is a 3d structure with immense complexity tightly packed and out of reach of any non-destructive probe, and the details matter. In fact, we don't know yet, to what extent they matter - or for that matter, which details. (Structural obviously. Electrical probably not, as that recovers from erasure. Chemical? Quantum? Answers are guesswork right now.)

The last person I read talking about how to scan a brain in detail was Anders Sandberg and his method would require death, brain removal, slice-n-dice, vitrification, microtoming and staining, and microscope scanning on an enormous parallel gang of microscopes, followed by computer assisted reassembly into a 3d model, and analysis so that neurons and their connections can be mapped.

Now imagine having all that data and trying to reverse it into a connected, living organ. Even if you could place all the cells and their wires that precisely, could you keep them in situ and oxygenated as you're printing billions and billions of individual pieces? And finish it before doomsday? And avoid it suffering the sensations of half-a-brain that is inexplicably not dying as it gradually grows towards sentience, for the duration?

Tongue in cheek: we know of simpler means of producing a person.

We don’t know enough about the brain yet to copy it, but what we do know would require reshaping each neurone to create the right synapses and dendrites and that’s not really 3D printing.

(While it is also notable that bio-printing is currently at the resolution of blobs off cells rather than individual cells, that’s not an in-principle objection in the way that post-processing 3.5nm resolution features into cells which are about 25μm in size is).

I assume you could do the gross structures eventually, but the real original neural state wouldn’t be there so not much point.