If the "Settle" button is grayed out, hover your mouse over it and it should give you an explanation as to my you cannot settle there. After playing Beta 4 a few times, I've noticed that places that are actually fertile are few and far between. Keep exploring with your troops and when you come across tiles that have the materials/grain and essence icons, those are the places you can settle. Everywhere else, you can't, even if there's a bunch of cool resources next to each other (if that's the case, create that outpost there and reap the benefits anyways).