Invalid Expression

I have created an expression to calculate the commision based on whether or not the Period Trxs are greater than the Period Sales Target.
The embedded (Sales Rev Target Attainment) expression is "SUM(CN_COMMISSION_HEADERS.TRANSACTION_AMOUNT)/MIN(CN_SRP_QUOTA_ASSIGNS.COMMISSION_TARGET)".

This is the invalid expression:

DECODE(SIGN(SUM(CN_COMMISSION_HEADERS.TRANSACTION_AMOUNT))-MIN(CN_SRP_QUOTA_ASSIGNS.SALES_TARGET),-1,(Sales Rev Target Attainment*CN_SRP_QUOTA_ASSIGNS.COMMISSION_TARGET*CN_SRP_QUOTA_ASSIGNS.INCENTIVE_MIX),(Sales Rev Target Attainment*Sales Rev Target Attainment*CN_SRP_QUOTA_ASSIGNS.COMMISSION_TARGET*CN_SRP_QUOTA_ASSIGNS.INCENTIVE_MIX)