When copying Nintex workflows from SharePoint on-premises to Office 365, Sharegate displays the following message:
Workflow action 'Action_Name' cannot be converted from on-premises to Office 365. This action will be replaced by a Placeholder action and the workflow won't be published. Please consult the help link for more information.
This error occurs because the source workflow contains actions that can not be converted by Sharegate from SharePoint on-premises to Office365. Sharegate will transform those actions into placeholder actions so the workflow can still be migrated. Placeholder actions cannot be published, so your workflow will be saved at the destination but not published.
The list of all the supported Office 365 actions is located here.
Simply go to the Nintex workflows section of Office 365. Open the workflow to edit it. The structure of the workflow will be there, simply replace the placeholder actions that Sharegate cannot convert to it's corresponding Office 365 counterpart. Here is an much more complete article on placeholder actions.