In order to read a spreadsheet file you must add the corresponding reader unit to "uses", in case of sfExcel8 (.xls) this is xlsBiff8, in case of the newer .xlsx (sfOOXML) this is xlsOOXML (or: fpsallformats for all built-in speadsheet formats). And you must select the correct format specifier in the wb.ReadFromFile call (or drop it for automatic detection):
uses
xlsOOXML;
[...]
wb.ReadfromFile('test.xlsx', sfOOXML); // or: wb.ReadFromFile('test.xlsx');
In your call your call ('ReadFromfile('tst.xlsx', sfExcel8)) you are forcing fpspreadsheet to open the file 'test.xlsx' as an xls file which does not work, of course.