Instead of using the Date_2[DateKey] column and fRedBox_ALL_BG[Month&Year] to built the relationship between the two tables, you'll need to two Date Type columns to create the relationship.
So you can firstly use the formula below add a new calculate column in fRedBox_ALL_BG table.
Date = DATE(fRedBox_ALL_BG[Year],fRedBox_ALL_BG[Month],1)
Then create the relationship with Date_2[Date] column and fRedBox_ALL_BG[Date].
And the [Sales Prev.Quarter] measure will work as expected.
Here is the modified pbix file for your reference.