DAX SUMMARIZE/GROUPBY Error: Multiple Columns Cannot be Converted to a Scalar Value

I am trying to group two columns together to group the number of products purchased on the same transaction.  I am using the summarize/Groupby functions to group the transaction number and the product name together.  Then I will use this information to see how many specific items were purchased on a single transaction (Units and Gross).  This is the formula I am using but I get the error mentioned in the title... I have tried everything and I get the same error.  I am also using this as a measure, and when I tried to build it as a column it was the same error.

Concession Groupby :=
SUMMARIZE (
    ConcessionSales,
    ConcessionSales[ConcessionTransactionID],
    ConcessionSales[ProductKey],
    "Concession Groupby", SUMX ( ConcessionSales, ConcessionSales[Gross Amount] )
)

Any help would be greatly appriciated!!