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.
Note: You will see this warning even if Show Item-Level is already checked. You will find the actual error(s) or warning(s) if you review the list items or documents within the affected list or library in the migration report.
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.