I've just added Nintex Workflow to a site and I'm adding a new workflow to a library in the site, however when I come to publish to workflow I get an error "Error opening a workflow. Value cannot be null. Parameter name: input".
Upon further investigation there is no Workflow Tasks list. Is this a list I have to create manually?
Yes. You need to create it manually. Then you need to choose it in your workflow (ex. with the "task" action), and then, during the publication process Nintex will enhance the tasks list provisioning there specific content type.