Enable SharePoint 2013 workflows in Sharepoint Online (Office 365)
In SharePoint Online, when creating a Nintex Workflow, the error message "Error connecting to workflow manager" is shown in Nintex workflow designer. This indicates that the workflow service store feature is deactivated and as a result the site has lost connection to the Workflow manager (SharePoint 2013 workflows). The SharePoint 2013 workflows option will be missing from the platform type when creating workflows in SharePoint Designer.
To enable “SharePoint 2013 workflows” for Office 365:
Navigate to Site settings.
Select 'Workflow settings'.
Select '(Workflow Health)'. (This is actually a link)
On the Workflow Health page, click on the 'Activate' next to the Workflow Service Store feature