Linking between tables in Numbers
It does work in iCloud Numbers:

Select the cell you want the result in: i.e  D2 of "Juice and Beverages"

And then in the text field at the top of the page enter:

=SUM('Units Sold' :: B2)


Take note of difference in how I have written it and how you did. Which was you main problem.





When done, click the green tick button on the right. And you will see this as the end result.





You can see some documentation on some of the included Functions by clicking into a cell and then into the:

1, text filed above:

2, you can search for a formula.

3,insert a Function template into the text field.

4, see a description of the Function and scrolling down further in the description you will see examples.