You must be signed into an account to make a farm or market. First make an account, then sign in. You should be redirected to your dashboard, otherwise you can navigate to it by clicking on the avatar icon in the top left corner. Once on your dashboard, click on the + button under Farms & Ranches or Markets, depending on your role.
If you are a producer you cannot add a market before you add a farm, because these markets should be only be added if they are managed by you,. Typically for markets on your farm or roadside stalls.