Cannot group that selection - Numbered results from a formula

Hello everyone,

Long time lurker, first time poster.

As the title suggests, I'm getting the "Cannot group that selection" error when trying to group a pivot table. I have searched and searched for a solution to my issue, but nothing has worked for me hence why I'm finally posting. Here's my situation:

I work in HR and am trying to create a Pivot Chart that will show how many positions are vacant, and for how long they have been vacant. If a position is vacant, the N column will show the date it became vacant. I then created another column to calculate how many days that position has been vacant for. Here's the formula I used:

=IF(N2<>"",NETWORKDAYS(N2,TODAY()),"0")

The idea is that when I group the numbers I'll be able to show the length of vacancy, and can filter out the positions that return "0", i.e. are not vacant. I know that I can manually group the numbers, but I'm setting this document up to automatically update as new positions will become vacant, and old positions will fill up, so I don't want to have to regroup everything every time we need to present the information.

One of the things I read about this error is that it can arise from having blank cells, hence why the [value_if_false] is 0. I also read that all the cells have to be the same format, so I have triple checked that cell formatting to ensure that they are all cells are set to General. Even with these measures, I'm still unable to group the numbers. What really confuses me is that I have already done something similar to this and had it be successful. The only difference with that Pivot Chart is that the formula which delivers the result (what the pivot chart is based on) looks like this:

=IF(AND(K2<>"9 - Complete",K2<>"9 - Cancelled"),NETWORKDAYS(B2,TODAY()),NETWORKDAYS(B2,P2))

Is there anyone who has any idea what my issue could be?

Thank you for your help!

After doing some more digging, I have found another difference that may be the root of the issue.

For the Pivot Table that allows grouping, the column that contains the equation is reading the results as numbers. For the Pivot Table that doesn't allow grouping, the column that contains the equation is reading the results as text. I found this when clicking on he Filter button in he column header. The former says Number Filter, the latter says Text Filter

For the column that's reading as Text Filter, I have converted it to Numbers, to General, tried to group each time and still am unsuccessful. Even after converting, the Filter option is still reading the results as Text and not numbers. I have verified that each cell is producing a numbered result. Any ideas how to fix this?

*** SOLVED ***

So I figured out what was causing Excel to be confused. For the following equation:

=IF(N2<>"",NETWORKDAYS(N2,TODAY()),"0")

When I put the quotation marks around the 0, it made the result a Text rather than a number. Once I removed the quotation marks, everything works perfectly fine. The final equation looks like this:

=IF(N2<>"",NETWORKDAYS(N2,TODAY()),0)