When you say it fails to start.... just 'clicks'.. do you mean the starter solenoid clicks but there is no engine cranking...?

Firstly do this to determine if the battery is faulty or not:
When it fails to crank turn the headlights on.. .do they come on quite bright....? If so attempt to crank...do they stay bright whilst trying to crank.... (even if the engine doesn't actually crank).
If they stay bright then the problem is probably not the battery... Turn the headlights lights off..

If when you try to crank they go very dim (or cut out) then yes the battery is flat... or you have a poor power connection. It may be the battery connections on starter solenoid, disconnect the main battery positive before checking them as it is very easy to short them to the chassis by mistake.

If when you crank there is 'nothing'... or only a 'click' check that the transmission is fully in Park etc, there may be a start lock-out switch preventing it from starting unless it is fully in park.

My guess is the starter solenoid... Let me know the results of the battery test..