Fatal error occurred

Hi,
After I start the vb6 program, a message box jumps out.

Code:

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FATAL ERROR OCCURRED! THE SYSTEM WILL SHUTDOWN!
System.InvalidOperationException: The Key 'xxxxx' does not exist in the
appSettings configuration section.
at System.Configuration.AppSettingsReader.GetValue(String key, Type type)
  at ssss.ProcessInitializations()

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Thank for advice.
Which file should I focus?

System.Configuration.AppSettingsReader