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What happens when you try to reset the iPhone?

If there is an error message, what is the exact

complete text of that error message?

You need to provide more information before someone may offer a solution.

Writing an effective Apple Support Communities question:

Writing an effective Apple Support Commun… - Apple Community

See if anything here helps.

Restore to Factory setting.  

How to factory reset your iPhone, iPad, or iPod touch - Apple Support