Aggregate functions are allowed in Aggregator transformations only.
A transformation that is not an Aggregator transformation contains an expression with an aggregate function. Since aggregate functions may be used in Aggregator transformations only, the expression is invalid. The aggregate function might have been passed into the transformation expression using a mapping parameter or variable.
Check the transformation expression to see if it uses an aggregate function. If the transformation expression contains mapping parameters or variables, make sure that their values do not contain aggregate functions.