You are correct that a negative standard deviation is mathematically impossible. It is likely that there is an error in your data or formula. Here are a few things you can try to troubleshoot the issue:

1. Double-check your data: Make sure that all of your data points are accurate and entered correctly. Check for any typos or missing values.
2. Check your formula: Make sure that you are using the correct formula to calculate the standard deviation. In Excel, the formula is
   Formula:

   =STDEV(range) 

   . Double-check that you have entered the correct range of cells.
3. Check for outliers: Outliers can greatly affect the standard deviation calculation. Look for any data points that are significantly higher or lower than the rest of the data. You may want to remove these outliers and recalculate the standard deviation.
4. Try a different method: If you are still getting a negative standard deviation, try using a different method to calculate it. You can use the AVERAGE and STDEV.S functions to calculate the standard deviation. The formula is
   Formula:

   =STDEV.S(range)/AVERAGE(range) 

   .

By following these steps, you should be able to identify and correct any errors in your data or formula and calculate a valid standard deviation.