Unlike other functions, one should call the aggregate functions ("SUM," "MIN," "MAX," "AVG," and "COUNT") either using angle brackets, or via the "Get" function (see the "Features of calling the aggregate function" section):

if <Sum(<Table1."Field1">, MasterData1)> > 10 then...

Another feature of an aggregate function is that it should be used inside the "Text" object; one can call it in the script afterwards. If the aggregate function is used in the script only (without using it in the "Text" object), an error message will appear. That happens due to the fact that an aggregate function must be connected with a definite band, and only then it would work correctly.