Uploading issue

hello ,
I am using the older grocery crud and I have an issue with the set_field_upload function.
I have defined the set_field_upload(‘file_url’,‘assets/uploads/files/’.$row->file_folder) ,
but when I try to upload a file , I am getting an error on upload using jquery.fileupload.
I don’t understand the reason.
the $row->file_folder is defined and has a value, the name of the folder. I have checked it.
When I defined the set_field_upload(‘file_url’,‘assets/uploads/files/’.date(‘Y’)'); works well.
Can you tell me what I’m doing wrong?

Thank you in advance.