My MacBook Air 2017 left command key doesn't work

if it was me then I would

1. try connecting an usb external keyboard to see if it would work on that
2. try to make another user or sign in as guest to see if the keyboard worked there

if it was a negative for both then I would assume a defect key

Not knowing what everything was in "I tried everything but replace the keyboard"

What does Keyboard Viewer show when you depress the Left Command key?

https://www.imore.com/how-use-keyboard-viewer-your-mac?

Try booting to Safe Mode. That boot up takes minutes and clears caches and other things that may resolve your problem.

How to use safe mode on your Mac - Apple Support

Still same problem in Safe Mode?

A lot of times just boot to Safe Mode followed by normal boot resolves problems.

If you really tried everything then you have a defective key switch and fix is to live with it, replace the keyboard or use an external keyboard if you want to. Your model has the older scissor switch which is better than the butterfly one covered by this program:

Keyboard Service Program for MacBook, MacBook Air, and MacBook Pro - Apple Support

Get expert service and support at the Genius Bar.

▷ Call 1-800-MY-APPLE or get help here. 