Go to: Setup → Sites → [Your Site] → Public Access Settings
Provide access to:
Apex Classes:
- UploadPageController
- AboutPageController
Flow Access:
- Enable access to the created Flow
Object Permissions:
Grant Read, Create, Edit access for:
- Account
- Contact
- Case
- Cloud Document (custom object)
Also, enable Field-Level Security for required fields.
For a detailed explanation, please refer to: https://satrangtech.freshdesk.com/a/solutions/articles/17000150864