Why wont my transmission engage the shifter moves but the truck goes no where

What is your question?

Transmission was just changed because of this problem and now with a new to me transmission in it still will not engage the transmission no gears rd123 non-working no indicator on dash board showing I shifted into gear. Fliud is brand-new and it is full. Please help or my rig will be towed.

What makes this problem better or worse?

Changed transmission and still same problem

How long has your 2003 GMC Envoy had this problem?

A few months