Enter layout mode and check the Table Occurrence listed in "Show Records From" in Layout Setup...
Now double click your fields while in layout mode and note the name of the Table Occurrence specified for this field. I predict they will be different.
Table Occurrences are the "boxes" on your relationship graph. If there is no relationship between the layout's table occurrence select in Layout Setup... and the table occurrence specified for the fields present on your layout, you'll get this error message. FileMaker is telling you: "I know there's a field and table out there, but I have no way to determine which record in that table should be supplying data for the current record of this layout because there's no relationship to use to make that determination."
Thanks! It works. And actually now that I have tinkered with it more it makes more sense. A graphic representaion is interesting and may be more intuitive for some.
And here we are, nearly five years later, and the answer still holds up. Just fixed my problem too---thanks!