This article will helps you to understand why device discovery is failing on SD Cloud
Device onboarding/discovery is failing with below error
Discovery PreCheck : Device with serial number FP.x.x.x.x.x already exists
If device discovery fails due to an existing serial number, follow these steps:
Verify that the device is not already discovered under a different SD Cloud organization (ORG).
If the device is managed in a different ORG, delete it from the old ORG and then attempt discovery in the new SD Cloud ORG.
If the device is not managed in any other ORG, contact the JTAC/SD Cloud engineering team to clean up the stale device entry.
After the stale entry is removed, delete and re-onboard the affected SRX device in the intended ORG.