I received the error message below when trying to publish a simple Nintex Workflow 2013 workflow with just a Call Web Service action. I was able to save the workflow with no issue, but not publish the workflow.
“The type or namespace name ‘Activities’ does not exist in the namespace ‘Nintex.Workflow’ (are you missing an assembly reference?)”
I was able to solve this issue by activating Nintex again on the web application via Central Admin -> Nintex Workflow Management -> Web Application Activation.