“The total items in a single folder exceed the 5,000-maximum threshold for SharePoint and OneDrive for Business migrations.”
On-Premises SharePoint, SharePoint Online, OneDrive for Business, and MigrationWiz
Cause of error
Due to SharePoint and OneDrive for Business threshold limits, MigrationWiz cannot migrate a document library that contains a folder with more than 5,000 items (files and folders).
For more information: Read the Manage large lists and libraries in SharePoint article from Microsoft.
To resolve this error, divide the folder containing more than 5,000 items into separate subfolders that each contain less than 5,000 items.