Filter the tasks which have not been accepted by any user

tp!assignees - List of users and groups who have been assigned the task. The object returned takes the User or Group data type.

tp!owner(Appian definition) -
When a task is assigned, the Task Owner property has a null value until the assigned user views the task or until a user in the assigned group accepts the task.
The Task Owner property can only be used to identify the user who completed a given task. If the task has been assigned to more than one user (if there were multiple users or a group of users) the Assignees property should be used to determine the users that accepted the task.

Other than tp!owner you can look into "task_assignee_owner()" - Assignee(s) before a task has been accepted or the task owner after the task is accepted.

And please check the security to the portal report, some times your user might not have permission to the knowledge center/folder where the ".arf" or Appian Report File exists.

tp!assignees - List of users and groups who have been assigned the task. The object returned takes the User or Group data type.

tp!owner(Appian definition) -
When a task is assigned, the Task Owner property has a null value until the assigned user views the task or until a user in the assigned group accepts the task.
The Task Owner property can only be used to identify the user who completed a given task. If the task has been assigned to more than one user (if there were multiple users or a group of users) the Assignees property should be used to determine the users that accepted the task.

Other than tp!owner you can look into "task_assignee_owner()" - Assignee(s) before a task has been accepted or the task owner after the task is accepted.

And please check the security to the portal report, some times your user might not have permission to the knowledge center/folder where the ".arf" or Appian Report File exists.