904 not shifting into gear

Ok, lets get simple first.
The neutral switch requires that the linkage is in the right place, and it has contact so it's just a linkage adjustment I'd bet.
Put the shifter in neutral and the trans in neutral with the lingage disconnected at the trans (just pop the rod off)
Then adjust the linkage to match where the rod lines up with the hole in the shift lever.

One other easy to eliminate possibility is that it does not have enough fluid in it yet.
(Chk it in neutral)

Then I would see if it has the right filter in it.
Then an air check on the clutchpacks and servo's
If you follow this in this order, you will find out real fast whats going on.

Just let us know if you need info on how to do any of this.