This error means that the ViewModelFactory is unable to find a suitable ViewModel for your component presentation. The most common causes for this are:
The root element name does not match the first parameter in the ContentModel attribute (it's case-sensitive)
The second parameter in the ContentModel attribute is set to false (you can set it to false for classes representing embedded schemas or linked schemas, but you must set it to false for component presentations)
The ViewModel class does not implement IViewModel
Also note that the root element names for your schemas should be unique, otherwise DD4T will never be able to find a matching ViewModel.