Hm. I finished 2.5 and never had such problem. The only thing I can think of is that you haven't researched everything you need to disassemble the UFO. IIRC you need Alien Origins, then when you get the UFO you should get the research project UFO Theory, this unlocks the research of the particular UFO type you have in yard, and only after researching that you can start disassembling UFOs of that type.