LED series Resistor- Noob question
The two LEDs in series have a forward voltage of 6v which is more than the Arduino's supply voltage.
Forward voltage is the bare minimum amount of voltage required for the LEDs to start conducting electricity (ie: to allow the current to pass through)
Read this: https://www.baldengineer.com/led-basics.html :

Forward Voltage
The Forward Voltage defines the amount of voltage
required in order to conduct electricity. Any voltages below this
amount will cause the LED to remain “Open” and non-conductive. This
also means any components in-series with the LED will not have current
flowing through them either! Once the voltage dropped across the LED
reaches the Forward Voltage, it will begin to conduct electricity. Not
only that, but the LED will only drop its Forward Voltage at any given
time.
For example, consider a LED with a Forward Voltage rated at 3.0V. Now
what happens if you attach the Anode to the Positive (+) Terminal of a
AA (LR-6) Battery and the Cathode to the Negative (-) Terminal? Will
the LED do anything? No! The AA (LR-6) Battery only has a nominal
voltage of 1.5V. Until you add a second battery, the LED will not
light up.