Yes, once we're commanding autonomous little bots in space, capable of also autonomously creating their own factories, we can build anything on a whim as long as we can design it and have the energy and materials.

(I may have played too much Factorio)

Harvesting that much material might be a challenge.

I guess you’d also have to build it in interstellar space to avoid planets and asteroids.

If FTL is possible, we didn't just missed something, but can throw all of our physics books on fire.