Summarize virtual table

Try

SummaryTable =
VAR Trans =
    ADDCOLUMNS (
        SUMMARIZE ( Transactions, Transactions[EffectiveDate], Transactions[AsOfDate] ),
        "@Cash Amount $", CALCULATE ( SUM ( Transactions[CashAmount $] ) ),
        "@WeightedTrans",
            DIVIDE (
                DATEDIFF ( Transactions[EffectiveDate], Transactions[AsOfDate], DAY ),
                DATEDIFF ( EOMONTH ( Transactions[AsOfDate], -3 ), Transactions[AsOfDate], DAY )
            ) * [@CashAmount $]
    )
VAR SumWeightTrans =
    GROUPBY (
        Trans,
        [AsOfDate],
        "@SumWeightedTrans", SUMX ( CURRENTGROUP (), [@WeightedTrans] )
    )
RETURN
    SumWeightReturns

The main point is to use GROUPBY rather than the second SUMMARIZE, but I've also tweaked the code a bit.

You should never use SUMMARIZE to add calculated columns, just use that for grouping and use ADDCOLUMNS to add the new columns you need.

I've also removed the Weight column as it wasn't being used, so there's no point calculating it.

Finally, I use @ in column names in temporary tables, so that they are easily distinguishable from columns or measures in the model.