Why would a 1997 Nissan Sentra not start and there be a clicking or buzzing from a 'unit' on the upper left of the intake manifold and the starter or solenoid won't engage but the battery is good?

I believe what you are hearing is the "Starter Relay" a device that sends voltage to the starter to turn. Two old tricks are (1) tap the starter lightly with a soft mallet or hammer, if the Bendix spring is getting stuck this may loosen it. The other is to use a screwdriver between the hot side and the starter side of the relay, they also make a starter switch that will do it. If it turns then your relay is bad. Good luck