Well, you should go into the settings of the list or library and scroll down and find the calculated column and ensure that the value is either ‘single line of text’ or a ‘date’. A single line of text is just fine.
Here is the formula that you need to copy and paste into the calculated column’s formula field. (Note the placement of the last and 2nd to last parenthesis.)
=IF([Start Date] > [Today], "Upcoming", IF([End Date] < [Today], "Expired"), "Due")
Basic logic explained:
If start date is less than Today, mark as “Upcoming”, otherwise, if end date is less than Today, mark as “Expired”, if neither condition is true, mark item as “Due”.
Your previous placement of the second to last parenthesis after your formula’s “Due” clause would have equated to:
If start date is less than Today, mark as “Upcoming”, otherwise, if end date is less than Today, mark as “Expired”, but if end date is NOT less than Today, mark the item as “Due”.
There are a couple better ways to write your formula, but you should be fine with my updated one laid out above.