How to Mark a List as Public
In order for the sign-up widget to work, there must be at least one list that is marked as public. In order to mark a list as public, which will allow a user to sign up to the list, go to the Subscribers tab then select "List Settings" for the list that will be made available for sign-up. The list settings are circled in red in the image below.
On the edit list screen (pictured below) check the box that says "allow user to sign up to this list" and then click save.
To enable sign-up to this list, go to Appearance > Widgets. Drag the "SendPress Sign-Up" widget to the desired sidebar and select the list that you made public underneath where it says "check off the list you would like users to subscribe to". Save the widget and visitors to your site will be able to sign up to receive email.