If I had my way, that message would read:
"This action cannot be performed because the required Tutorial: What are Table Occurrences? cannot be accessed."
Your layout refers to a specific "box" in Manage | Database | Relationships--a table occurrence. This table occurrence is specified in "Show Records From" as found in Layout Setup... for your layout. When you add fields to your layout using either the field tool or field picker, you also select those fields from a table occurrence. The names in the drop down at the top of these dialogs are the names of the table occurrence boxes found in Manage | Database | Relationships.
If there is no relationship linking your layout's table occurrence to that from which you are selecting a field for the layout, you will get this error message as FileMaker needs that relationship in order to determine from which record in the related table to use in order to show and save data in/from that field object on your layout.