Sometimes when we load a family into a project, Revit spits out a can't make type error and does not import anything.
This error is typically caused because of "0" values in some of the parameters. For instance, if someone starts creating a family from a template and does NOT use some of the parameters like width. This family would be a non parametric family. Revit assigns the value zero to these unused parameters but a Revit project cannot accept the zero value for certain parameters...(width, etc.) So, Revit does NOT load this family into the project.
To rectify this, open the family and then the Design bar > Family types. In the dialog box, select the unwanted parameter and delete it. After deleting all these unwanted parameters, the family should load properly (and since the family is NOT parametric, deleting these parameters should not affect the family too.)