I've just started The Molecular Level mission and I chose to go through the BoS. I just built the stabilized reflector platform, but it doesn't show it as built. It still has the hovering marker above the workshop. I went to try to talk to Proctor Ingram and she just gave me her generic responses. I really don't want to go back to my last save because it's an entire 5-6 hours behind back at the Cambridge Police Station when the Prydwen first arrives. If anyone has any fixes for this, that would be awesome.
Your structure is not built properly.
The first piece is the Stabilized Reflector Platform. After that piece is built, you will then finish the rest of it and the Beam Emitter is suppose to snap on top of the Platform. However, it is possible the platform is sunk into the ground and the Beam Emitter does not snap to it.
You need to store the Platform, build a wooden floor, build put the Platform on the wooden floor, then try snapping the Beam Emitter on it.