You cannot convert a live "Buy Now Only" listing into an auction (or vice versa); you must relist the item to change the primary format.
You can often add or edit a Buy Now price on an active auction if certain conditions are met: trade me add buy now
If you don't want an auction, choose the "Fixed Price" format. This creates a listing with only a Buy Now price and no bidding. Adding Buy Now to a Live Auction You cannot convert a live "Buy Now Only"