This is due to a Microsoft limitation on Excel spreadsheets and there is no way for Spotfire to avoid this. The most you can export to Excel is 1,048,576 rows by 16,384 columns. See
Excel specifications and limits for more details.
If you require an export to Microsoft Excel from Spotfire, ensure your data has fewer than 1,048,576 rows and fewer than 16,384 columns in Spotfire before exporting.
Note: this row limit only exists for exports to the Excel file formats - there is no such limit for the other available export formats, such as .csv or .txt files.