Unlocked obliterator behemoth but cant build it

[Dimmer]

I have everything needed to specialize but when notification pops up telling me to accept nothing happens, the screen pops up telling me what i will need then when i click it saying yes it just returns to the base galaxy view.

[Groo the one]

You'll have to wait some turns, for to get your specialization finished. Check out the Behemoth you did choose for upgrade. It should be indicated when hovering over the Behemoth how long it takes for upgrade.