Apple Watch Band

Replace yes, free of charge, unlikely. Any reason you think it should be free?

What band is it? What exactly is it missing? Can you provide a picture?

Unless you can prove it was a manufacturing defect, it is not covered by warranty. If you have Apple Care+ for the Watch, it will use one of the accidental damage incidents you have which have a cost, albeit a reduced one, associated with it.

If it's a brand new band, and you think it's missing something, you can take it to Apple and they'll examine it and explain your options to you.

click here ➜ Apple Watch Service & Repair - Apple Support

if it's under warranty and apple don't judge it to be user caused damage the warranty should cover it

read

Legal - Hardware Warranties - Apple

The clip button. It would only be free, if it's determined to be a manufacturing defect.

Take it to an Apple Store and have them look at it, they can tell you your options.