How to deal with 'Error in formula'?

What does the error mean?

It means that a formula is incorrect or invalid.

Example:

In the cell N25 the formula SUM() is spelled incorrectly as SUMM() which returns the error (and additional errors of linked cells).

How to fix:

Change the entire formula to improve validity. Use the Excel help if needed

Also see here for best practices to assess formulas in financial models / business cases.