With “Root Directory” he meant the Website Folder. From the first File Manager I see there is an htdocs folder, so it’s there!
Try to upload the website files again, but don’t use the File Manager to do so if there are too many files or the website is too heavy; instead use an FTP client (here is a guide for setting up one).
If it still gives an HTTP ERROR 500 after doing that, perhaps your WordPress installation is corrupted:
And when you update WordPress, take note of this as well: