Rolling 12 Financial Periods

DAX DAX Calculations
12

Without seeing your Dates table I’m hoping the Month Offset is a non-repeating sequential number…
Can you give this a go

Sales 12P = 
VAR myOffset = SELECTEDVALUE( Dates[MonthOffset] ) 
VAR ListMonths = CALCULATETABLE( VALUES( Dates[MonthOffset]), FILTER( ALL( Dates ), Dates[MonthOffset] >= myOffset -11 && Dates[MonthOffset] <= myOffset ))
RETURN

    CALCULATE( [Total Sales],
        FILTER( ALL( Dates ),
            Dates[MonthOffset] IN ListMonths
        )
    )

I hope this is helpful