Hi can I sale brand new Apple iPhone X at Apple store

No. If it’s under contract, you have no legal rights to sell until you’ve discharged the terms of your contract. If you attempt to sell it privately, the contract holder can charge you with theft, and you’ll also still owe them for the terms of the contract.

If you are unable to meet the terms of your contract, you need to discuss that with the contract holder.

On contract from where? If it is on contract it is not yours to sell. Turn it in to the people who you contracted with.

As the others have said, it's not yours to sell. It belongs to the company you contracted with.

On top of that, Apple Stores do not buy phones.