Certain reusable workflows can be configured to require the presence of specific columns in the lists or libraries they are associated to. These association columns are generally added using SharePoint Designer 2010 and up.
When migrating your workflow from your source site, your association columns do not appear to have been created at the destination site.
When migrating a reusable workflow, association columns are not automatically migrated by Sharegate at this time. This feature has been deprecated beginning in SharePoint 2013.
The association columns must be manually re-added to the workflow post-migration.