Buy plan apple care+

How many days since you purchased your iPhone? You had 60 days from date of original purchase to add an AppleCare+ Plan. If you are past that date, it's too late to buy one now. If for example you purchased your iPhone on launch day, you would be past 60 days from original date of purchase at this point.

Then stedman1's advice to contact Apple directly is the right advice. We are all users here on this user to user only forum. There has to be a reason AppleCare+ isn't an option on the phone, but as users, we would not know why.

I think this link will take you to AppleCare+ SA --> AppleCare Products - Apple (SA)

From Apple directly, or from a reseller, as in an actual Apple Authorized Reseller, like a carrier or like BestBuy in the US? If you bought it from a non-authorized seller, the warranty started the date it left Apple or their authorized sellers inventory and would not start the date you bought it.

So provide as many details as you can about who you bought the phone from on October 11th.

Also, where in the world do you live and did you purchase the phone in the country you live? If for example you purchased the phone in a country different from where you live, you wouldn't be able to add an AppleCare+ plan on the phone from outside the country you live.

As the option is not shown on the phone, even though it appears to be eligible for AppleCare +, it would be advisable to contact Apple directly.

From Apple directly, or from a reseller?

You are welcome.