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If you are locked out of your S6, then you will have to factory reset it to get back in. Before that, try entering some incorrect passwords (if asked) or try using your fingerprint to unlock multiple time and it might ask you for the Gmail account and associated with it. So enter it and you will be able to reset the password.
If it doesn't ask for Gmail, then your only option is factory reset unfortunately. Factory reset will erase everything from it. This is how Samsung designed it.
Follow the steps below to factory reset your Galaxy S6
Turn off the phone
Press and hold down Volume Up + Home + Power buttons at the same time. Android system recovery will be loaded now
Select wipe data/factory reset. (Volume buttons to go up or down, select with Home/Power button)
Select Yes — delete all user data.
Then reboot your phone This will factory reset it. While setting up, use the Gmail account that you used on it before and your contacts, calendars etc should be restored.
If you have any questions or would like any additional information, please don't hesitate to ask.
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