SUM function can allow you to add values(numbers or dates) in a table using a calculated or text field but the calculated or text field will be outside the table, not inside the table in an app as you can see in the following link.
In this case, for “Values” to sum up, please specify the code of a “Number” field, the formula that results in a numerical value, or a number.
For example, to sum up values in the Subtotal fields, whose field codes are “AMT_C” of a table, the formula in a calculated or text field is SUM(AMT_C).