The names of the fields have nothing to do with this. The table occurrences (boxes) in Manage | databae | relationships and how you have linked them in relationships are what you will need to examine to figure out what is going wrong for you.
Each layout specifies a single table occurrence in Show Records From in Layout Setup. That's your starting point. Each field and/or portal you add then requires you to first select a table occurrence (Filemaker labels the drop down as though this is a list of tables but the names listed are of the table occurrence boxes--not the actual tables) in a drop down, then you specify the fields taken from that table's table occurrence. Since it's possible to have more than one table occurrence for the same table, you might have added fields from an unrelated table occurrence and this will produce the error text you are seeing.
To learn more about table occurrences and how they control the function of your databases, see here: Tutorial: What are Table Occurrences?