You will need to edit your membership types to be available online for purchase or renewal.
Go to Functions -> Memberships -> View Types and 'Search';
View the columns Member Can Renew Online and Non-Member Can Purchase Membership Online.
Determine which of your memberships you would like to sell or be available for renewal and enable the setting.
Also, make sure there is a document assigned for signature under Assigned Document.
Assign a document (Windom Membership Agreement) and Save.