When running a migration, you get an indication to:
Show item-level filter option to display the details
This warning occurs on a list or library. The warning indicates that items within the list or library have errors and you need to have Show Item-Level checked in Filter to review these errors.
From the Migration Report, click Filter, and select Show Item-Level under Options.
Once done, you will be able to see details regarding errors or warnings on list items and documents within the list or library.
Note: You will see this warning even if Show Item-Level is already checked.