Deleting columns changes the dimensions of the table which can cause a forward going loop to fail. Try to delete from the end.
dt = Current Data Table();
n = N Col(dt);
// Delete last six columns columns, starting from the end
For(i = n, i > n - 6, i--,
dt << delete columns(Column(i))
);
// This should also work: delete very last column six times
For(i = 1, i <= 6, i++,
dt << delete columns(Column(N Col(dt)))
);