At this section you can find information about your migrations history and if your Jira Cloud is ready for migration
(Settings → Apps → Migrations Dashboard)
You can sort that list by any column and you are also able to see migration details by click on Name of migration of your choice. Then you will be moved to details page where you can see Projects that has been migrated and Execution logs where you can see steps of migrations and also potential warnings and errors that occur during migration.
On migration details view you are able to see extended error reporting so once anything go wrong with your migration you are able to see what exactly happened here.