iPhone 6s vibration not working

Did you already try to force restart your phone?

Force restart iPhone - Apple Support

Setting it back to factory settings, without using a backup afterwards, would be the next step after that, before getting it serviced:

Restore your iPhone, iPad or iPod touch to factory settings

Apple - Support - iPhone Repair and Service

sayyed17 wrote: "38%"

sayyed ~ If you got that number from here:

Settings > Battery > Battery Health > Maximum Capacity

...then the weak, aged battery is probably the reason why the vibration isn't working. Have the battery replaced:

iPhone Battery & Power Repair

About genuine iPhone batteries

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sayyed17 wrote: "Battery replacement price"

The price in $$ is here:

iPhone Battery & Power Repair

...But if you're outside the U.S.A., Apple's worldwide phone #s are here:

Contact Apple for support and service

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I could not find any information about the costs for a battery replacement on the iPhone Service Page for India:

iPhone Battery Replacement - Official Apple Support (IN)

You might have to call them to get more details about it.

Find the number for your location

Check this:

...

...

copied from: iPhone Battery Replacement - Official Apple Support

Two things

1. Settings > Accessibility > Vibration [Enable]
2. For email
   1. You will have to set it independently for ever mail account (Watch the GIF below)
   2. Note: - Default is No Vibration.
   3. 

sayyed17 wrote: "I AM FROM INDIA"

Okay...

Apple - Find Locations in India

Contacting Apple in India

Or search for India here:

Contact Apple for support and service

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Switching the country settings from United States to your country of choosing in the right bottom corner will also work right away.

sayyed ~ If your iPhone 6s still has the original battery, perhaps it's too weak to vibrate. What's the % value here?...

Settings > Battery > Battery Health > Maximum Capacity

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