Engine not cranking over?

Could be just a bad battery. Jump start it and take it to one of the parts stores that tests/replaces batteries. If it tests bad install a good one and then have the charging system load tested to be sure it was only a bad battery.

It does sound like the battery is failing. Remove the battery and have it tested. Most parts stores will charge and test the battery for you at no charge. Good luck and let us know how it goes.

Did it crank but would not "catch", or what did you hear? How did you check the battery and alternator when the engine would not start?

Next is to check if you are getting fuel at the fuel rail, and to make sure that you have spark at the plug wires.

Check that you have twelve volts at the small terminal on the starter when you turn the key. If yes, the starter will be faulty. If no, you will have to test the ignition switch and the transmission range switch. This is the switch on the transmission that stops the car starting in gear.

First measure the battery voltage. It should be 12.6 volts if it is fully charged. If it is closer to 12.2 volts it is okay but discharged. Charge it, then measure the voltage again while the engine is running. It should be between 13.75 and 14.75 volts. If it is low we will have to diagnose the charging system.

If the battery is fully charged but the starter still will not crank the engine, first observe whether you hear a rather loud clunk from the starter each time the ignition switch is turned to "crank". If you do, suspect worn contacts in the starter solenoid. If not, one suspect is the neutral safety switch which is part of the range sensor.

With the flickering you described, you might want to look at where the two smaller battery wires connect and be sure those connections are clean and tight. The negative wire bolts to the body. The positive wire bolts to the under-hood fuse box.

Check items in picture. Then try resetting security system.

I would make sure the battery has a full charge and the connections are good. Any fuses you check, you need to use a test light, see if the fuse circuit is hot.

Do you think the security system might be causing an issue? Are the circuits at the starter relay okay? Any testing at the relay, make sure the transmission is in park or neutral and the e-brake is set.

No crank at all/nothing when key is engage to starting position.

Could be a blown fusible link, the starter, starter relay, clutch switch/park and neutral switch/transmission position switch and ignition switch assuming the battery condition and connections are good.

https://www.2carpros.com/articles/service-engine-soon-or-check-engine-light-on-or-flashing

Have the computer scanned for code/s-you have a problem within the engine management system that caused the CEL to turn on. This is your starting point of diagnosis, finding out what is going on.