Hi @ahmed.cherif_drc.ngo , I can’t quite see the full formula or error message, but if you’re referencing a subform field from the parent, you’ll need an aggregate function.
This is because, referenced from the parent, a subform field doesn’t just have one value, it could have many (or none).
If you do want exactly one value, you can use the FIRST or LAST function. You’ll have to provide a second field to tell the evaluator how tell which record is first and last. This is often a date field.