Why can I not attack my farms, they only show a reinforcements option?

If you have the reinforcements option then the farm city is in your alliance. You cannot attack members of your own alliance.

You can either get them to leave the alliance, attack them and then let the rejoin. Or you can set up a Farm alliance that you and other alliance members keep your farms in. One that you treat as an ally and defend from others but you can still attack bases in it when needed.