Get in car, put key in, turn to start.
If starter does not engage, relax the key to "ON", wait 45 seconds.
If the engine cooling fan is running at high speed, it is the RCM. End of discussion.

If engine cooling fan is NOT running at high speed, it can be a low battery, but it is very likely an interlock issue.
Interlock issues vary between automatics and manuals, as they have differing criteria to engage the starter.

Determine the outcome of the above test before proceeding.