I just tried it in Crusade for v2.8 opt-in. For Terrans, the Xeno Research Lab is under the Xeno Experimentation tech.
I have no problem researching it, nor building it.
Now, I checked in the Base Game for v2.8.
Seems like the start building for research (which should be named "Research lab") is no longer available in the Colonization tech. As Xeno RL is an upgrade for that, you can't build XRL on it's own.
This is a SHOWSTOPPER BUG. The Research Lab needs to be available in the Colonization technology, just like Basic Factory is.
The problem is not in 2.7. It's solely in the 2.8 opt-in.