You need to include the INDEX function for it to work as I believe you intend.

Table.Quantity references ALL values of that column, meaning it is a set of values not a single value, but your calculation seems to be designed to multiply the quantity and unit price for a single row.

The INDEX function allows you to reference a specific value from that set of values, and you can use the ROW() function within a table or collection to reference the "current" row.

As a result, your calculation should be as follows:

=MULT(INDEX(Table.Quantity,ROW()),INDEX(Table.Unit_Price,ROW()))

This calculation would multiply the quantity and unit prices only from the same row as the field containing the calculation.