To access files in base general folder, you may not specify the drive Id as below. https://graph.microsoft.com/v1.0/sites/{site-id}/drive/root:/{path-to-folder-encoded}:/children

But to get the files from a non base folder, that is not in 'shared documents', you need to use, https://graph.microsoft.com/v1.0/drives/{drive-id}/root:/{path-to-folder-encoded}:/children