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.

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.

However, the 48 ohm resistor can combine with the 2A current source to form a Thevenin equivalent in the following way, that is correct:

^{simulate this circuit}