T-SB-0105-11 Mentioned here, not your 2012 but same issue of P0012 at Sienna Chat forum;
P0012 code
Depending on how you want to proceed, I'd do another oil change and give your self time to have a new replacement oil control valve (VVT Solenoid) ready if having to remove the intake manifold. Cleaning the oil control valve may work if that's the issue, but being there is a possibility it could just need replacing I'd suggest have the replacement ready. I recall hearing oil control valves can be finicky, to summarize a characterization. Of course I have likely skipped some other trouble shooting steps, proceed at your discretion.
There isn't much room to work so you may want to resign yourself to removing the wiper cowl, blue tape to mark wiper placement and it all comes out fairly easily as seen.
The support bracket at the back of intake manifold, passenger side is tricky but doable. Should be similar to the 2010; remove wire harness bracket near strut tower, left arm goes through here to the back fire wall where bolt is for guidance. Right hand then comes down with 14mm, might be 12mm, socket to
universal joint adapter and probably a 4" or 6" extension right about where you'll see a steel line going down the fire wall. Your left hand can guide socket onto bolt, you might want to wrap tape around the universal joint to keep it a little more rigid. My older universal joint was rigid but the second time I was in there I used one that wasn't so rigid and kept twisting but still worked...eventually. Your results may vary, seeing a couple of YouTube spark plug video tutorials is highly recommended. Highlander or any other Toyota or Lexus 3.5 models are going to be similar enough, as I recall it...
Just to add an observation, spark plugs and coils are made difficult by the wire harness being strapped down so tight I skipped the valve cover gasket thinking I'd never get the valve cover off. It probably is doable if you understand all the hold downs for the wire harness, not in my page of knowledge though...If I can recall any other details I'll post it.
I did change a oil control valve on a Corolla 4 cyl (actually Vibe) that seemed to have successfully cured it's ills as I remember, I sold the car soon after and the next owner eventually had the struts replaced so I'd think it was still working good for him. Car Care Nut on YouTube may also be a good resource for more information.