Scp to Synology DSM file system will use SFTP, not through SSH(default port: 8022), so you should use port 9022 (default port for SFTP) instead. And, you should use user space file system root as the target root. For example, you have a shared directory on /volume1/books/, you can use /books/ as the path, /volume1/books/ not works.

scp -P 9022 convert_to_utf8 username@synology_dsm_hostname:/books/collection/