Issue
Access Denied When Saving Workflow Templates or Snippet even though the user account appears to have appropriate permissions to publish list workflows.
Resolution
Grant "Contribute rights" to the NintexTemplates library at the site collection level (https://YourSiteURL/NintexTemplates).
Additional Information
Nintex Workflow Templates and Snippets are saved into a hidden library at the site collection level, requiring "Contribute" permissions at the site collection level. When a user publishes a Site or List workflo0w, the workflow definition is saved in the NintexWorkflows hidden library. Each site (or subsite) contains it's own NintexWorkflows hidden library.