Hi Roxx-3623,
Below blog shares a tutorial about how to connect to sql server based on Stored Procedure using bcs, you may take a reference:
You may encounter permission issues after the completion of the preceding procedure. In this case you need to set the permission for the external list. Go to the SharePoint Central Admin -> Manage Service Application -> Business Data Connectivity Services and select the newly created external content type from the list to set permission.
Best Regards,
Baker Kong