Hey, I appreciate there are no feedback signals so it is very difficult to know what is actually happening! There is a current parameter right now where you can't build a settlement that is within 2 hex tiles of another settlement, I think this might be the issue happening. So you're weren't doing anything wrong it's just lack of onboarding from us as we have focused on building systems as opposed to a tutorial up until now! Really appreciate you checking out the game and giving feedback even in it's unfinished state :D
Edit: You can only build a settlement that is 3 hex tiles away from other settlements and not within 2 hex tiles of any settlements