Append requires a "display box" and does not recognize a Data Table as such. Try the dt << New Data Box() command to capture a table within a display box. Or if the goal is merely to browse the tables, a plain journal could work.
Examples:
nw1 = New Window("vertical tables");
nw2 = New Window("horizontal tables", hlb = H List Box());
For(i = 1, i <= N Table(), i++,
nw1 << Append(Data Table(i) << new data box); //Active tables on top of eachother
hlb << Append(Data Table(i) << new data box); //Active tables side by side
Data Table(i) << journal; // Journal; tables inactive, for browsing data only
);