If you are seeing the below error when uploading an Excel workbook containing a data model, you need to toggle a SharePoint Online Feature.
"Excel Online - This workbook contains external data connections or BI features that are not supported."
The feature you need to toggle is the Preview Features.
“The preview feature is an IT control feature for SharePoint Online to globally shut off access to all services where data could leave the SharePoint Online compliance boundary. This includes data that would go to Power BI. This is an IT override. When preview features are disabled Power BI is disabled as are features in Excel Online that depend on Power BI technology to deliver the feature.
If an organization wants to have strict adherence to the SharePoint Online compliance boundary they can set Preview features to disabled. If they make this choice certain features are disabled including Power BI and Excel Online features that depend on Power BI.”
Go to Admin
Preview Features > Enable preview features