How do I get the sum of a field in a sub-form

I am completely new to Zoho Creator. Using the SUM documentation below, I attempted to use a formula field to sum a single numeric field in a sub-form. I could never get it to work. I then tried with all fields in a db with only one form. Still no go, and I got the same errors.

Should sum() work in the Expression property of a Formula field? If not, where do I use it to get a total for a field? If the sum function is not the way to go, what is?

Here are results of various entries in Expression property

Form - TestForm

Numeric Field - NumberIn

Formula Field - SumNumberIn

1 Numberin Works, but of course it only shows each record's value, not a total

2 sum(NumberIn) "Not able to find 'sum' function"

3 Testform.sum(NumberIn) and SumNumberIn = Testform.sum(NumberIn) "Expression not supported in formula"

Sum Function

//to sum the total value of a numeric field in the form
<variable> = <Form_Name>.sum(<Numeric Field Name>);

//to sum specific records with a given criteria
<variable> = <Form_Name>[criteria expression].sum(<Numeric Field Name>);

where,
<variable> refers to a user defined variable name or a form field.

Example:

//to find total sales value
Total_Sales = FormA.sum(Amount);

//to find the total sales value for Item="Printer"
ItemA = FormA[Item == "Printer"].sum(Amount);

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