Please enter a unique number to identify the task XXXX

I am trying to copy the existing project along with its task where i am facing this issue Please enter a unique number to identify the task. The length of task_number and task_name is according to api standard. Below is the script for task assigment. Can you please help here if i am missing anything

for task_rec in get_task_data(project_id) loop
a := a + 1;
l_tasks_in_rec.pm_task_reference := task_rec.task_number;
l_tasks_in_rec.task_name := task_rec.task_name;
l_tasks_in_rec.pm_parent_task_reference := '';
l_tasks_in_rec.task_start_date := '';