The error message you are seeing is telling you that for the numbers that you entered, there was nothing left over for personal miles.  The business miles + commuting miles was greater than the total.  So, the program assumes that the error is in the number of commuting miles. 

Revisit the section where the miles was entered to be sure all of your numbers are accurate, but remember the total must be greater than business + commuting. 

The assumption is that:  total number of miles driven - business miles - commuting miles = personal miles.