IIRC the Craft Laser Cannon is a BEAM class weapon (as opposed to a CANNON), which means it can only be used on the Darkstar, Lightning, Firestorm, and Avenger. It's in an odd spot because unless you manage to rush lightning/firestorm you won't really have anything to mount it on for quite a while.
Craft weapon classes should be detailed on the weapon ufopaedia page, and weapon mounts available should be detailed on the craft's ufopaedia page.
Plasmas should be BEAM class as well.