Diagnosis
The camera is notoriously picky about SD card format and type. It doesn't like cards formatted by other devices (like a computer) or cards that aren't high-endurance models.
How to Fix It
First, power the camera down completely. Remove the SD card. Insert it back in until you feel a firm click. Power the camera back on. If the error persists, the card needs to be formatted by the camera itself. Go to the camera's settings in the app: Storage > SD Card. Tap "Format." Let the camera do the work. If there's no format option, the card isn't being detected at all.
If That Didn't Work
Your SD card might not be compatible. The TrackMix requires a microSD card (not a full-size SD with an adapter) and works best with cards 256GB or smaller. Crucially, it needs to be a card designed for continuous video writing—look for "High Endurance" or "Surveillance Grade" on the label. A standard cheap card from a phone will fail quickly.
Heads Up
Even with a compatible card, constant 4K recording fills it fast and wears it out. The "SD Card Error" can be the first sign of a card dying from write-cycle exhaustion. If you keep getting errors after a few months with a non-endurance card, that's likely the cause. Consider using the Reolink cloud or an NVR for primary recording.