Serial numbers can only be maintained for items assigned to a plant

What causes this issue?

The item has not been assigned to a <GL:plant>plant</>.

System Response

The system does not allow serial number assignment for items which are
not assigned to plants.

How to fix this error?

If you want to maintain <GL:serial_number>serial numbers</> for this
item, enter a plant for the item.

Error message extract from SAP system. Copyright SAP SE.