If the manifold somehow has a gap of air that leaks through from maybe an improper install, can this cause the code.

P0446 is A vent valve code (located near gas tank/charcoal cannister). If intake plenum was recently removed and reinstalled you may want to check the line going to the purge valve (located on intake plenum). This code sets basically when pcm commands vent on and purge valve to open and the fuel tank pressure sensor sees no change. This can be due to A large leak (gas cap off,line disconnected/broken,faulty gas tank pressure sensor,or vent valve failure(this is A common failure if vehicle is driven in A dirty/dusty enviroment).