When you configure the "Assignment Task" action in your workflow designer, there are 2 required fields you need to set. (1) the assignee of the task (who this task will be assigned to), and (2) the task form realted to this task (this is the page that the task owner will see when check the task information).
If you are not set the assignee for the Assignment Task, you will receive the error message like below:
This means the Assignment Task with name "Assignment Task_1" does not have the assignee to be set.
To resolve this issue, please go back to the workflow designer, select the Assignment Task, and set the assignee in the settings panel by clicking the "Assignment Editor".
The Assignee can be a user, a job position, or based on the workflow expression etc.