The reason you are getting an error when using the MONTH function within the SUMIFS statement is because the MONTH function is not a valid argument for the SUMIFS function. The SUMIFS function only allows for range/criteria pairs to be specified.

To achieve the desired result of summarizing the amounts by Category and Month, you can add a helper column to your Detail worksheet that extracts the month from the DateOfService column using the MONTH function. Then, you can use this helper column as a criteria in your SUMIFS function on the Summary worksheet.

Here are the steps to add a helper column:

1. Insert a new column next to the DateOfService column.
2. In the first cell of the new column, enter the formula =MONTH(DateOfService).
3. Copy the formula down to all the cells in the column.

Now that you have a helper column, you can use the following formula in your Summary worksheet:

=SUMIFS(Amount,Category,A2,HelperColumn,1)

where HelperColumn is the column you just created that contains the month extracted from the DateOfService column.

This formula will sum the amounts in the Amount column where the Category matches the value in cell A2 and the month in the HelperColumn is 1 (January). You can copy this formula to the other cells in the summary table to summarize the amounts by Category and Month.