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Before clicking following button to generate token, you have to have facebook application Read documentation here and follow how to create application and set all fields of application properly.Else you may get errors also listed in the documentation
Some users get this error - "Given URL is not allowed by the Application configuration.: One or more of the given URLs is not allowed by the Apps settings....." , it is a error because of not setting "Site URL" value in facebook application basic setting.Please follow the last image shown in the documentation here
Click the following button to generate access token entering your application id and secret key