Hello Vinoda,
Typically this error means that the delivery that you are attempting to open is deleted. Typically if you are trying to open a delivery activity when this occurs it usually means that you copied and pasted a delivery activity from another workflow into a new workflow and the activity is still referencing the previous delivery.
If this is the case, the recommendation would be to create the delivery from scratch and not use copy/paste as it can be rather problematic.
Regards,
Craig