Following error you may see during migration.
Collect vCenter log bundle and review migration-assistant.log, you may see following line.
2019-05-21 16:52:51.229Z| migration-assistant-10964411| I: ValidateExportDir: Required core space: 5648; core, events and tasks space: 6466; All space: 168668; FreeSpace: 13952; 2019-05-21 16:52:51.306Z| migration-assistant-10964411| W: ValidateExportDir: Export directory C:\Users\adminuser\AppData\Local\VMware\Migration-Assistant\ does not have enough free disk space to migrate all database data. Required: 168668 MB, Found: 13952 MB.
This error message comes when the source vCenter events/logs size is higher than destination vCenter. The solution is to select option Configuration when it comes to select migration data.