Anonymous users, i.e. users who do not have a user account in your frevvo tenant, cannot save their partially filled forms to be completed later. This is because frevvo needs to know the identity of the user who saved the form to bring it back later. Without that there is no way to know which saved form belongs to which user.
See this documentation about anonymous sign up. Using this feature, anonymous users can register to your tenant and create a user for themselves. Once the users sign-up, they can then use the Save/Load feature in frevvo to save their partially filled form to the task list and open it again later to complete it.
Take a look at this frevvo Space that one of our customers is using. Notice the Signup menu (under Important Items) that the users can use to sign-up into this tenant.