How can I activate the opacity tab on opaque or masked material?
You click the material properties and mark it translucent.
The whole point of opaque and masked, is that they don’t have varying opacity.
This is because of how the hardware works: Blended triangles can’t use direct Z buffer tests, and thus you need a different mode for those. This also means that blended triangles may end up with sort order problems, depending on how the art is constructred.