Why can't I build factional units?

[desert]

Yes, but there are more than 3 starting cities.

[Hax]

You haven't built level 4 governments in there, right?

[Maion Maroneios]

Actually, it's training not building. And it has to do with the government types you have there. Only lvl1 (for sure) and lvl2 government buildings give you the ability of building factional MICs which, in turn, allow you to train factional troops like Principes and Triarii.

Maion