Why can't I change the "Summarize By" (sum, count, average) section of a calculated field in a Pivot Table?

[datafriend]

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!