I got a MacBook Air but the owner left it signed out.

If the original owner did not do this then the computer is of no value to you. Get the original owner to complete the steps in the link: What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

All you can do is try:

Erase your Mac and reset it to factory settings - Apple Support

Unfortunately with a used Mac, you have no idea if it was previously managed by MDM software that would prevent you from resetting it and set it up as yours.

If this is a used Mac that was not prepared properly for a new owner, you might also encounter Activation Lock or a firmware password. You cannot bypass either, and since you are not the original owner, Apple will not help you to remove either.