Port face selector is missing its source object.
A port face selector is an internal intermediary object between a structural slab and one of its bounding structural members.
Currently, the source object of a port face selector is limited to a structural member.
The source object has been deleted, but this delete operation was not propagated to the port face selector. Or, the relationship between the port face selector and its source object was deleted for an unknown reason.
The slab limited by this member will not correctly evaluate boundary offsets.
What You Should Do
Because this object is internal, there is nothing that you can do directly.