Created Unassigned: Read Excel [9474]

I am trying to read excel and convert it to DataTable but getting error in first statement itself

var workbook = new XLWorkbook(filePath);

or

var stream = File.Open(filePath, FileMode.Open, FileAccess.Read);
var workbook = new XLWorkbook(stream);

Exception: Value cannot be null. Parameter name: source

Any help?