As the warning suggests, if you want to use ColumnFormat your Data can't be a table. Try using a cell array for Data, as in:
pl = struct2table(app.Preset);
app.Preset_UITable.Data = table2cell(pl);
Then you can set ColumnFormat to include a cell array of chars representing the items in a drop-down menu.
I'm not sure how many columns your table has (the screenshot shows 4, but the ColumnFormat/colu you define has 2) nor which column is intended to contain the drop-down menus. Make sure ColumnEditable is set to true on the drop-down column. You should avoid naming a variable cat, since that's the name of a built-in function.
app.Preset_UITable.ColumnName = pl.Properties.VariableNames;
RankMode = Function_Categorical_Struct("RankingMode");
cats = categories(RankMode);
% 4 columns, 3rd is drop-downs; adjust as needed:
colu = [{'char'} {'char'} {cats.'} {'char'}];
app.Preset_UITable.ColumnFormat = colu;
