they took out the garden, so you can only get food from a food resource now. I am not a big fan of this new system myself.

I had this problem also, even after researching harvesting, farming & advanced farming. A second game had similar problems for a while, though eventually I was able to build a farm. It was unclear what eventually allowed it.

Some way to check what buildings we cannot yet build & why would be nice, because I'm still unclear whether there was some reason I was being prevented from building a farm.

Play as Altar, farms work fine.

Some of the other factions seem a bit broken; no matter what research you have or positioning you choose the options to build just aren't available.

I can build farms as Altair so long as I have something finished next to the resource.  

I actually preferred being able to build in your territory for certain resources though even if it was a bug.  Typically large farm lands and mines aren't in city walls.  I think things would be better like that if there was an option to raid those buildings to capture resources if they weren't defended.