Hello, beginner here and working with 2 LED lights and messing with circuit layouts, wires etc. When I use the 9volt battery the lights won't light up. When I use the USB, they do light up. Isn't 9 volts of DC enough to light 2 LED?
9V block batteries are for very light duty, like smoke alarms, and cannot supply enough current for a bunch of LEDs, motors, etc. They will run an Arduino, but not for long.
The clear blunder is not comprehending what the "Vin" or "RAW" terminal is. The regulator on the Arduino UNO/ Nano/ Pro Mini/ Mega2560/ Leonardo/ Pro Micro has very little heatsink, so will not pass very much current (depending on the input voltage and thus, how much voltage it has to drop) before it overheats and (hopefully reversibly) shuts down. It is essentially a novelty provided in the very beginning of the Arduino project when "9V" power packs were common and this was a practical way to power a lone Arduino board for initial demonstration purposes. And even then it was limited because an unloaded 9 V transformer-rectifier-capacitor supply would generally provide over 12 V which the regulator could barely handle.
Nowadays, 5 V regulated switchmode packs are arguably the most readily available in the form of "Phone chargers" and switchmode "buck" regulators are cheap on eBay so these can be fed into the USB connector or 5 V pin to provide adequate power for most applications. Unfortunately, many tutorials or "instructables" are seriously outdated or misleading and have not been updated to reflect the contemporary situation.
If powering from batteries, as long as the battery pack cannot exceed 5.5 V, this must be connected to the 5 V pin.