Help & FAQ/Import guides
Standard Notes offers a decrypted backup export, and that is the file PrivacyNotes reads. One important detail up front: it must be the decrypted variant, not the encrypted one.
An encrypted Standard Notes backup is ciphertext that only your Standard Notes password can open, so the importer cannot read it. If you exported the encrypted variant, export again and pick decrypted. Since the file contains your notes in plain text, delete it after the import is done.
Drop the whole .zip into the importer, or just the .txt file if you already unzipped it.