1. Here's why

The only models in the People and Animals category that support Transparency are the models on the People - 3D - Silhouettes and Animals - 3D - Silhouettes tabs:

The models on the other tabs do not currently support Transparency as this functionality has not been implemented for these model types:

2. Workarounds

2.1: Use models from the People - 3D - Silhouettes and Animals - 3D - Silhouettes categories as these model types support Transparency:

2.2: Alternatively, if you can't replace your people/animal models with models from the People - 3D - Silhouettes and Animals - 3D - Silhouettes tabs, consider rendering the Photo, Clip or Movie twice: one version with the people/animal models and the other version with the models placed on a hidden Layer.

The version with models:

The version without models:

2.3: Then in an image editor or video editor, add both images as layers and blend the two by adjusting the transparency of the one with people/animal models:

External Link: Three Ways To Blend Two Images In Photoshop

Example of blended image: