Details
App webs are special SharePoint sites that contain components used by SharePoint 2013, SharePoint 2016 and Microsoft 365 apps. ShareGate Migrate cannot connect to those sites nor list them as subsites under another site. ShareGate Migrate doesn't migrate app webs since each app would need to be handled differently.
Workaround
The best way to make sure everything works well is to deploy the app manually at the destination after the migration.