With the save, I was able to confirm that the reason you can not build the Behemoth is that your lack of Adamantian should prevent you from queuing it in the first place.

I've passed this along to the team.

any chance this could be related to the tractor beam cost bug back from supremacy beta?

https://www.games2gether.com/amplitude-studios/endless-space-2/forums/114-bug-reports/threads/34322-carrier-tractor-beam-strat-adamantium-cost?page=1#post-285321