Clean WordPress installation is recommended + latest Divi parent theme should be active.  Please verify permalinks are enabled for API endpoint to work. Settings -> Permalinks -> Post name Install plugin...

"Divi Sync" is designed to work with Divi 4 Theme Builder header and footer. For upgrade purposes files can be downloaded and imported via Divi -> Theme Builder. https://dashboard.cmsbased.net/dl.php?type=d&id=82...