Getting an error when trying to release a production plan.

Summary: Made a production plan under constraint conditions and released the plan.

Some items were released as Firm and others as not firm for testing purposes to check output.

The error received was as below

"The supply recommendation couldn't be released for supply type Planned Order supply because the value provided for the PLANNER attribute isn't valid. The current valid is NULL."

Checked again in the plan inputs tab and the item has an assigned planner.