Do you have two sets of filters available on your source? You summed up Audio/Video Filters, but there is also Effect Filters:
Apply LUT, Chroma Key, Color Correction, Color Key, Crop/Pad, Image Mask/Blend, Luma Key, Render Delay, Scaling/Aspect Ratio, Scroll, Sharpen, Virtual Cam
If you do not have Effect Filters, what (kind of) source do you want to apply the Mask/Blend filter to?