As mentioned above, check that you have selected row 1 as headers when loading. If you still have an error, check that there is a value in the header row cell, for every column that has data in it. It's possible your spreadsheet may have data somewhere in a column without a header row value. To check this, select and copy the EXACT data area into a new workbook. Then try importing that. Sometimes ServiceNow detects the remnants of old data in columns that appear to be blank, and if the header cell is empty it will throw this error.
