why subtotal is not valid number?

[gawa]

@JingyuWu 

Because you try to sum up Subtotal that is "String" field. Convert it to numeric field before summarize tool.

Why the option "sum" appears even though String field is, I guess,  Subtotal was previously Numeric fields and sum-up by Summarize tool but you must have changed field type to String during workflow development, and configuration in Summarize tool still remains. 

Anyway, try to change field type to Numeric(integer or double or float) so as to correctly sum-up values.

[JingyuWu]

Thank you for your reply. But I still cannot solve the problem. Because I can only choose a few options from the screenshot.

[alexnajm]

I would use a Formula tool to create a new numeric column with the function ToNumber(ReplaceChar([Subtotal], “$,”, “”))

you need to clear out all of the symbols that are non numeric in nature, and THEN it can be numeric and you can sum the values

[TimN]

You need to remove the $ sign and comma from the number I think.  Here's an example