Problem adding statistical formula [UPDATED]

Problem adding statistical formula [UPDATED]

UPDATE: I was able to see the entire popup using Mozilla Firefox and then Zooming out the screen. However my only option in the left panel is just the "Add Aggregate Function"

Hi!



I would like to report this incident. I cannot add an statistical formula, probably because there is a field that I need to fill in above the popup. I cannot scroll up and down to see what are the choices. I just see this exact screen (at the bottom) and I cannot view whether there is an option to "Add Statistical Formula" from the Left Panel. I am not sure. The error that I get whenever I click the OK button is: 
Sorry, cannot create a Aggregate Formula. You need to have an Aggregate function used in the formula.

For other types of formula, use "Custom Formula" option.


Might be because I clicked "Add Aggregate Formula" in the left panel that I cannot add a statistical formula. 
Where do I find the "Custom Formula" or "Statistical Formula" option then?