After I convert to Excel in the DAQEXPLORER viewer, I can't open the file.

Check the number of data. Excel's maximum worksheet size is 65536 rows by 256 columns. If there are 65536 or more data along the time axis, the file cannot be opened in Excel. For example, if 1 second of data data is taken every second for 1 day, that totals 86400 data which exceeds the maximum. When converting to Excel, specify an appropriate time width to convert.