The SAP message FL 389 can also appear if you are assigning a structure which contains a string field to a function module tables parameter. Within an RFC enabled function module it shows this message as a success message but will not let you save.
You get a slightly different message with none RFC enabled functions modules but has the same effect “<TABLE> must be a flat structure. You cannot use internal tables, strings, references, or structures as components.
Solution
Using the function module tables parameter is obsolete so the simple solution would be to create a table type of your structure and then create a EXPORTING or CHANGING parameter based on this.
Create table type via transaction SE11
Create EXPORTING/CHANGING parameter
