Excel 2007, and the versions that have come after it (365, 2010, 2013, 2016, 2019) like to run with workbooks having an extension of xlsx, xlsm, and/or xlam.
Earlier versions of Excel, such as the very popular Excel 2003, generally expected to create and run with workbooks having an extension of xls.
Excel 2007 and later versions usually have no problem at all with "old" xls workbooks.
When an xls workbook is opened with Excel 2007/2010/2013, Excel will say that it's running in "compatibility mode". The screen snapshot shown below exemplifies what's seen at the top of an Excel window when Excel 2010 is running in compatibility mode.
At times, Lertap will have a problem or two with compatibility mode. For example, when the "Item scores and correlations" option is taken, Lertap will sometimes fail. In such cases, a message such as the following may appear:
Sorry, there's been a problem inverting the correlation matrix ....
As far as Lertap goes, it is best to avoid running in compatibility mode. If you've opened an xls workbook with Excel 2007 or later, save a copy of it as an xlsx workbook. Then close Excel completely. When you return to Excel and open the saved xlsx copy, all should be well -- compatibility mode should not be running. Invisibly, Lertap will thank you.
For more assistance with this problem, please feel very free to write to firstname.lastname@example.org.