My left AirPod isn’t working.

See this support article:--> If your left or right AirPod isn't working - Apple Support

If the issue persists, I suggest that you make an appointment (*) and have your iPhone+Airpods inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> https://locate.apple.com/choose-country-region . After checking it out, they'll give you all the available options.

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.