Where is the cramksahft position sensor

It's on the backside of the engine, under the intake manifold. It's held on by one 10mm mounting bolt. Like so many sensors, access is the big deal. You may have a better shot at it from below.

Crankshaft Position Sensors and Camshaft Position Sensors are notorious for failing on Nissan Altimas. I would replace both of them--in fact, they use the same sensor.