The SharePoint library is not visible in the 'Get Document Reference -Specify Library' step in the Workflow.
Before You Begin
The issue can possibly be caused by a Site Title rename. K2 artifacts are generated and placed into a category name similar to the SharePoint Site title. After the site title is renamed, the artifacts will be generated and placed in the category with the new site name.
You can resolve the issue by renaming the Site back to the original name then recreating your library and SmartObject.