The GetLabwareBarcode function only returns one barcode value at a time based on wherever the current position of the sequence is set. It does not return an array of all barcodes found in that sequence.
So, you would get all the barcodes by looping over a sequence corresponding to the tubes (making sure the current position is set to wherever you want to start collecting barcodes). Then use the GetLabwareBarcode function to get the position’s barcode from the current sequence position and set the value into an array. Increment the sequence by 1 per loop iteration and loop until the sequence is consumed.