I just bought a 15" MacBook Air 16gb 512gb and I did NOT set up a password... for 2 weeks I open the cover and I was in... today...for no apparent reason a password is required... since I never set up a password, how can I enter one... fix???

Hmm. A brand new Mac, on first power-on, would take you through the steps to set up this computer. One of the first steps is to create a new user account, and associated password that you decide on.

Did this not happen? If so, then I would suspect that they sold you a returned & repackaged product that whomever had it did not remove their Apple ID from it.

You basically have two choices, should this be true. Return the laptop back to Amazon and get a full refund. Alternately, before doing so, contact Apple Support directly and see if they can assist you with this issue. To do so, simply select the "Support" link on the lower left-hand side of this forum's web page.

To my knowledge it has not been possible to create a new User Account without a password for a long time now. I could be wrong about that, but I just tried creating a new User Account without a password and was unable to find a way around that requirement.

It used to be possible in the past though. When you initially set up that Mac, did you migrate its content from a previous Mac? If that previous User Account was not set up with a password, that is one possible explanation. In that case, just leave the password field blank. Click Return or OK or whatever is asking you for a password.

Related: I don't know how to unlock all my passwords on my mac - Apple Community

A couple things missing in your post is the following bits of information that would be helpful in providing you with a solution are:

• Did you purchase a brand new laptop? ... or a used one?
• Did you purchase it directly from Apple or an Apple Authorized Reseller? ... or from some other 3rd party?