Without looking over your shoulder it is not easy to tell if this is a glitch/bug or if it is operator error.
Start a brand new document, enter a few numbers in the table, make a chart, and see if you can add custom error bars to it. If you cannot, you may be doing something wrong or expecting something different than what Numbers does.
EDIT -- Before you do that, try this: Did you select the series or is the entire chart selected? The series must be selected before you can put custom error bars on it.
Without looking over your shoulder it is not easy to tell if this is a glitch/bug or if it is operator error.
Start a brand new document, enter a few numbers in the table, make a chart, and see if you can add custom error bars to it. If you cannot, you may be doing something wrong or expecting something different than what Numbers does.
EDIT -- Before you do that, try this: Did you select the series or is the entire chart selected? The series must be selected before you can put custom error bars on it.
The boxed to input the data or select the data do appear but if you click them nothing happens, press tab and you don't go anywhere, or even click on some data so as to select a range of cells for the custom data and you are simply selecting the tble again.
It is as if the input boxes don't exist.
Have you tried it?
EDIT:
Your edited suggestion worked a treat - thank you so much!