iPhone 6s not charging

See this support article:--> If your iPhone or iPod touch won't charge - Apple Support

If the issue persists, it might have a battery or other internal components problem. Make an appointment and have your device inspected at an Apple Store or Authorized Service Provider.

Regards

Giulio

BuNnYmOmMy ~ If the battery of your iPhone 6s is the original one, it quite likely needs replacement:

iPhone Battery & Power Repair

About genuine iPhone batteries

(Articles found by searching Apple's Knowledge Base.)

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Check the link in blue above your name on this page. If that doesn't help, maybe it's time to make an appointment at the nearest Apple Store for an evaluation and explanation of your options...

https://www.apple.com/retail/

Contact apple.

For the 'iPhone Service Answer Center' click here --> https://support.apple.com/iPhone/repair/service