Hello, just to expand the answer given by José.
Every record in your entity Comment needs to know to which record in the entity Passenger it is related. So you created a Foreign Key (FK) in it, called PassengerId, and defined that it is mandatory (and this makes sense).
As this FK is Mandatory, in the moment you want to create a new record in the Comment entity, you need to provide to which passenger this comment is associated. You probably have the PassengerId input parameter, as this seems to be a DETAIL page, that has this value. So, before SAVING (Create or CreateOrUpdate action) in the action that saves this record into database, you need to add an Assign node and set the PassengerId attribute of the record you are using to store the comment.
If you don't provide a value, the database will accuse an error because it will try to locate in table Passenger the ID, that is not being provided, and will not find it.
I imagine that this Comment entity is an extension of the entity Passenger (the Primary Key is of type PassengerId), as it seems that only a single comment can be entered.
Cheers.