i have no option to turn on two factor authentication on my iphone 7

See Two-factor authentication for Apple ID https://support.apple.com/en-us/HT204915  See this section especially Setup two-factor authentication for your Apple ID. From this section, "Two-factor authentication is available for Apple ID accounts with at least one device that's using the latest iOS, iPadOS, or macOS. Learn more. You can follow these steps on your iPhone, iPad, or iPod touch to turn on two-factor authentication." 

Note that, even though the latest iOS, iPadOS, or macOS devices are mentioned above, sometimes two-factor authentication is not available. "While most iCloud users can now turn on two-factor authentication, certain account types may still be ineligible at Apple’s discretion." See Availability of two-factor authentication for Apple ID https://support.apple.com/en-us/HT205075

You would know only if you don't have the option to turn it on. All you can do to try to find out what Apple is thinking is to contact Apple Support:

Get a call from Apple support to help solve your problem using this

link https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362

Apple ID Support https://support.apple.com/apple-id

Apple ID account security for country-specific and

region-specific help https://support.apple.com/en-us/HT204169

Get Support: We’re here to help https://getsupport.apple.com/. Go through the options to find the best support option.

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