Chroma key masking does not require a mask file. Instead, you set the Stencil texture’s HSV Color Key parameters so that a certain color, or a certain range of colors, are either visible or invisible.

Chroma key masking has no effect if an image file is mapped to Mask. Make sure there is no Mask file assigned to the Stencil texture (type a space in the Mask field) if you want to use chroma key masking.