Opened a brand new Macbook Air M1 that I had bought 2 years ago and the Battery won't charge.

The one year warranty starts at the date of purchase and not when the computer is first used. So, if you had it for two years and never used it, you are out of luck.

FWIW, if a battery is left for that long and has never been charged, it is likely dead and will need to be replaced and you will have to pay for it as the warranty has long since expired. Rechargeable batteries do not like being left to sit discharged for long periods of time, like two years. It did have some charge when leaving the factory but some circuits are always active and will have drained the battery long ago.

BTW, Best Buy is now an Authorized Apple Service Provider and should be able to replace something like a battery without shipping it out. Then again, that may also depend on your local Best Buy and they may not do it in store but send it out anyway to a repair center.

I believe this is covered under the unlimited 1 year warranty?

Check Your Service and Support Coverage - Apple Support will answer that question.