If you move or delete an assignment file from disk, and then open the InDesign document from which it was created, InDesign won’t know where the assignment file is located. You need to re-create the assignment file so InDesign can find it.
Select the missing assignment (it will still be listed in the panel) and choose Update Selected Assignments from the panel menu.
Choose Update All Assignments from the panel menu.
Select the assignment and choose Change Assignment Location from the panel menu. Specify a location and name for the new file.
Double-click the assignment name. In the Assignment Options dialog box, click Change and specify a location and name for the new file.