You need to simultaneously select 8 cubes of the same type (either standard or tactical, not a mix of both). Also some missions may not have the full techtree unlocked. Test it in a skirmish game to be sure.

Yeah I know it has to be 8 Cubes, I have tried it with classic and tacticals, that button to merge them is missing. Theres also no technology to research related to Cube fusing and no building. But I don't really remember if there was any requirement.

Tried skirmish and indeed option to merge them is there. In campaign mission 8 - still nothing.

No, I mean like in other RTSes, things get unlocked gradually as the campaign goes along, as in they are intentionally blocked by the devs for that mission.

I know but I have completed 8 missions from 10. I remember fusion was there sooner than that. I just tried to look at mission 9 and no fusion either. Seems different than it used to be. I know I could use that sooner than in last mission.

As I haven't seen the correct answer to this question:

The Fusion Cubes are only available in the last Borg mission. It was also only in this mission in the oribinal game. Although you could have used it outside the single player missions in skirmishes and PvP maps as far as I remember.

And you need to build a fusion hub as well, which is part of the technology assimilator, where you also can extract assimilated technology by dismanteling a cube, that assimilated technology.

As far as I remember, in Armada 2, one can make Fusion Cubes only in the skirmishes, and not in any single player missions. I am unaware of any way to unlock Fusion Cube tech for the sp missions. If anybody does, please do let us know. Thanks.