Here, Build dataTable activity didn’t connect to the start and please check once.
And also store Build Data Table will give dataTable as output. Then use output DataTable activity to convert dataTable to string.
You need to change the data type of the newDT to DataTable. Then use that variable in the output field of Build Data Table and Input for Output Data Table.
Try to hit control k then type your variable it will automatically create a variable with a data type of datatable.