That is not true. There are circuits that are neither parallel nor series. Here’s a classic example: Take four resistors of various values, and connect them to form a square. Make another square with four more resistors. Now, while holding one square above the other, connect four more resistors, each one going from a corner of the bottom square to a corner of the top square. You just built a cube. Finally, connect one wire to any corner of the cube, and a second wire to its diagonally opposite corner, and apply a voltage source. Current will flow in all twelve of the resistors. But you cannot resolve the cube into a single resistor by combining series or parallel components. Not one of the twelve resistors in the cube is in series with any other, nor is any in parallel with any other.