I can't use WhatsApp by my Apple phone 6s

Hey there,

See>>>Update your iPhone, iPad, or iPod touch - Apple Support

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if you having trouble with update >>>If your iPhone, iPad or iPod touch won’t update - Apple Support

I can't use WhatsApp

Check iPhone type -> Go to Settings -> General -> About.

WhatsApp for iPhone requires iOS 10 or newer and has been discontinued as of November 1, 2021 with support for Apple iPhone 4 & 4S

iOS 9.3.6 is the latest iOS version for iPhone 4S.

When you have an iPhone 6S, see lafeef response.