When migrating a Nintex workflow to Office 365, the following message appears :
Workflow action 'action name' cannot be converted from on-premises to Office 365. This action will be replaced by a 'Placeholder' action. The workflow will be saved at the destination but won't be published. Please consult the help link for more information.
Sharegate supports an array of Nintex actions but some of those actions are still unsupported. The unsupported actions will be replaced by a placeholder action. This action is simply an empty action with the title of the source action. Here is a list of supported and unsupported actions.
You will need to edit the workflow at the destination. Simply delete the placeholder action and replace it with the corresponding Office 365 Nintex action. Here is more information about the placeholder action.