How to select a specific cell in UITable?

UITable.Data can return any of the following types: table array, numeric array, logical array, cell array, string array, or cell array of character vectors. If you run the following command you will be able to tell what type of data you are working with:

uit = app.UITable.Data;
class(uit)

"app.UITable" implies you are working within App Designer. Within App Designer, components/functions/properties specific to the app are called with the prefix "app.". It's just a syntax error, so if you add the prefix to you code that should allow you to interface with the UITable.