You need to change your profile to the "advanced listing" tool.
Go to the eBay home page - and click "sell" at the top right of the screen. Then, click "advanced" on the top right of the next screen. eBay will prompt you asking if you're sure you want to switch - yes, you want to switch. Now all of your listing layouts will have everything, including the "best offer" checkbox.
I found the above when I googled your question. Hopefully it will work for you.