My note is on the SD card and Relay can't open it. Why?

Relay reads your handwriting through Browse & Access, and that only shares notes stored in your Supernote's internal storage. Anything on the SD card is not offered over the connection, so Relay can't fetch it, convert it, or attach it to a task — this is a limit of the device's sharing, not something Relay can work around. If you keep notes on the SD card deliberately (a common reason is keeping private files off the cloud), the tidiest setup is to turn off Link notes to tasks in Settings → Task apps → Linked notes settings, so Relay stops trying and stops asking. Tasks still push normally, just without the note.