Can't I change the "Summarize By" (sum, count, average) section of a calculated field in

I'm learning to use PivotTable and I'm attempting to create calculated fields and change them to counts or sums, but it's greyed out? No matter which data source I use, how simple the calculation is etc. I wanted to change Sum to Average, so to get average profit and average cost per age group, but I'm really unsure why it isn't working.

I made a simple, irrelevant dataset just to see if my original data was too complex to do it, and it still doesn't work. But I'll post the fake dataset that I made, and the greyed out aspects.

I'm on Excel 365 for mac for reference.

Thanks in advance for all the help, this has been keeping me up all night!

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