You have 4 opening parentheses and only 2 closing, so you should close the other two as well.
You should outline your expression a bit, to not get lost in how it nests:
Sum(
IIf(
[SKU Breakout 004]![Year_Week] >= [z Calendar Reset]![Year-Week-Disco]
AND [SKU Breakout 004]![Year_Week] < [z Calendar Reset]![Year-Week-Ship],
IIf(
[SKU Breakout 003]![Distribution Type] = "DELETE",
0,
[SKU Breakout 003]![SKU_Cust_Mix_Current%]*[SKU Breakout 004]![Qty],
IIf(
[SKU Breakout 004]![Year_Week] < [z Calendar Reset]![Year-Week-Disco],
[SKU Breakout 003]![SKU_Cust_Mix_Current%]*[SKU Breakout 004]![Qty] ,
[SKU Breakout 003]![SKU_Cust_Mix_Future%]*[SKU Breakout 004]![Qty]
)
)
Now you see clearly that there is something is missing. Maybe you need this:
Sum(
IIf(
[SKU Breakout 004]![Year_Week] >= [z Calendar Reset]![Year-Week-Disco]
AND [SKU Breakout 004]![Year_Week] < [z Calendar Reset]![Year-Week-Ship],
IIf(
[SKU Breakout 003]![Distribution Type] = "DELETE",
0,
[SKU Breakout 003]![SKU_Cust_Mix_Current%]*[SKU Breakout 004]![Qty]
),
IIf(
[SKU Breakout 004]![Year_Week] < [z Calendar Reset]![Year-Week-Disco],
[SKU Breakout 003]![SKU_Cust_Mix_Current%]*[SKU Breakout 004]![Qty] ,
[SKU Breakout 003]![SKU_Cust_Mix_Future%]*[SKU Breakout 004]![Qty]
)
)
)