Using Measure to create month/year calculation, works in one query but not the other

I'm trying to create a Month & Year calculation and thought I was suppose to use Measure. I have two different queries one for Sales transactions and another for Reversals transactions. Forgive me as I’m a newbie and learning Power BI:

Oddly, in my sales query it works fine but in my reversal query, it gives me this error

"A single value for variaton 'Month' for column 'reversed_at_PST' in table 'All_AMS_Reversals' cannot be determined. This can happen when a measure formula refers to a column that contains many values without specifying an aggregation such as min, max, count, or sum to get a single result."

This is the formula I’m trying to enter for my reversals table report:

Month & Year Reversals = 'All_AMS_Reversals'[reversed_at_PST].[Month] & " " & 'All_AMS_Reversals'[reversed_at_PST].[Year]

Whereas the one in the sales table report works fine:

Month & Year Sales = 'All_AMS_Sales'[created_at_PST].[Month] & " " & 'All_AMS_Sales'[created_at_PST].[Year]

When I try to modify or edit the Measure formula, it doesn’t autopopulate with all my available fields, it only shows two? I think this might be where the source of my problem is starting? I should have a long list of fields to select from?

I confirmed they are both data type: date/time and the 2nd query is basically a copy of the 1st query so it should have all similar settings. I have picked the year and month in my hierarchy under fields. I’m not sure what I’m missing and why it works in sales but not the other?

Thanks so much!