My left airpod does not work and when I put it back in the case the green light flashes. What does it mean and how to fix it?

Apple hasn’t documented what flashing green means, but it seems to be tied to a failed AirPods earpiece.

Try a reset: How to reset your AirPods and AirPods Pro - Apple Support

If the reset does not clear this, contact Apple Support. Use the Get Support link in the upper right of this webpage.