If your vehicle is showing engine won't crank at all (just single or rapid clicks), here are the most likely causes and the parts you'll need to fix each one.
The battery lacks sufficient charge or cold cranking amps to spin the starter motor. The rapid clicking sound is the starter solenoid repeatedly engaging and disengaging as the voltage drops under load.
Corrosion or loose clamps create high electrical resistance, choking off the massive current needed by the starter. This is a frequent issue after recent battery replacement or maintenance if terminals aren't fully tightened.
The starter motor's internal brushes are worn out, or the attached solenoid has burned contacts. A single, loud click usually indicates the solenoid is engaging but the motor itself cannot spin.
The relay that switches power to the starter solenoid is failing or has burned contacts. This can cause a weak click or rapid chattering sound coming directly from the fuse box rather than the engine bay.
The starter engages with a single heavy clunk, but the engine physically cannot rotate due to catastrophic internal mechanical failure (like seized bearings) or fluid trapped in the cylinders.