Hi Sonia!

Not all calculations are available for fields inside of tables. Here's an easy way to accomplish what you need.

Go to the field where you want the total to be and open up the advanced tab. In the calculation field, enter the following:

=SUM(MINUS(VALUE(INDEX(TABLE_NAME.COLUMN_NAME, 1))), VALUE(SINGLE_LINE_VARIABLE))

Make sure you replace TABLE_NAME.COLUMN_NAME and SINGLE_LINE_VARIABLE with the correct variables using the variable picker. 

Because you're using single line fields, you'll need to use the VALUE function to get the numerical value from the fields. 

For a list of formulas that work in tables and lists, check out this help center article and scroll down to "Functions for Tables and Lists"

Let me know if this works!