What is the error "case sensitive file system" mean n how do i fix to migrate

It means files stored such as Text.rtf will be seen as different than text.rtf

Case insensitive file system both files Text.rtf & text.rtf will appear as the same file.

For Migration the Formats should be the same type, though case insensitive file system is recommended for the best for Apps working as expected.

So, either some filenames will need changing or reformatting the drive.