engine won't start Nissan 4

https://nissanantitheftsystemway.evergrovio.com/posts/understanding-your-nissan-anti-theft-system-the-key-symbol-and-what-it-means

My Nissan won't start this morning—just a clicking sound when I turn the key. I checked the battery terminals and they look clean, but the lights are dim, so maybe the battery is just too old. Could also be the starter motor or a bad connection. I'll grab a multimeter to test the voltage before calling a mechanic.