Your KYC can be rejected for multiple reasons, including:
1. Blurred images
2. Face not clear in selfie or Govt Id
3. Face in selfie no matching with photo in Govt Id
4. Not using original document, etc.
You will be able to find the exact reason when you restart the KYC process from My Ads > Preview Ad page (please refer answer to question "How can I start my KYC?")