enter doesn't work with sumif

I have created a workbook with several sheets. The first sheets are a list
of components in different sub-assemblies. The last sheet searches the other
sheets for a total of components used. When I generate the sumif formula, I
can see the result in the formula window, but when I press enter, the formula
is displayed and not the result.

Make sure the format of the cell is not text then re-enter the formula.