You must have placed the config.xlsx file at same location where your main.xaml is, which is the entry point of your project.
If you place the config.xlsx file in Data folder, which is by default the structure proposed by UiPath Re-Framework, then you have to call it using either Environment.CurrentDirectory + "\Data\Config.xlsx" or simply "Data\Config.xlsx".
Main directory is always where your Main.xaml file is.
By default, as per Re-Framework, while invoking InitAllSetting.xaml, argument is "Data\Config.xlsx"