Every circuit transformation that you do must give a result at the "A & B" terminals indistinguishable from the final Thevenin or Norton equivalent: for this circuit arrangement you must have an open-circuit output voltage of 48V, and a short-circuit current of 4A at the A & B terminals for every step.
Your attempted circuit transformation gives an open-circuit voltage of 96V and a short-circuit current that is infinite:
simulate this circuit – Schematic created using CircuitLab
Another problem - I could combine 48 ohms and 2A current source this way, which would also be wrong:
simulate this circuit The primary reason why these reductions don't work is those pesky output terminals A and B don't allow you to do transformations at that end...a Thevenin or Norton equivalent circuit has TWO terminals, not FOUR.
