If Excel cannot correctly evaluate a worksheet formula or function, it displays an error value (such as #REF!, #NULL!, or #DIV/0!) in the cell containing the formula. The error value itself, plus the Error Options button that appears in cells with error formulas, help identify the problem.
Excel Error and Solution | #NA #DIV/0! #NAME? #NUMBER! #REF! #NULL!
Please note: The information in this article applies to Excel versions 2019, 2016, 2013, 2010, 2007, Excel for Mac, and Excel Online.
Excel displays a small green triangle in the upper left corner of cells with error values. The green triangle indicates that the cell contents violate one of Excel's error checking rules.
When you select a cell containing a green triangle, a yellow diamond button appears next to the triangle. The yellow diamond is Excel's Error Options button, and it contains options for correcting the observed error.