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Are you using Data belnding technique here? if so this is the reason.
Sorting by Field Is Unavailable for Data Blended Measures:Sorting by Field Is Unavailable for Data Blended Measures | Tableau Software
If you want to do this you can use a hack:
Create a calculated field based on the measure that would return the right alphanumeric sort, such as -SUM([Sales]) for a descending sum of Sales, then put that as a Discrete (blue) pill to the left of the dimension you want to sort, and finally turn off Show Headers for the -SUM([Sales]) header.
If Dataset A is your Primary data source you should be able to sort on the sales no. measure.