Is the gear display fully highlighted , that is all gears displayed at once? If so the transmission is in "limp mode". At this point I would scan the vehicle to retrieve any fault codes stored. You can go forward with a diagnosis from there.
Make sure the fluid level is correct , double check the installation , i.e drive shafts , shift cable , torque converter/drive plate and the like.
If this a new transmission and it is an internal failure you may have a warranty.
As an aside it is VERY EASY to damage the oil pump at the front of the transmission if the torque converter is not properly seated when the engine/ transmission are bolted together......