Apologise and be specific
Name the import and say it did not complete.
Say what went wrong
Give the real cause in plain words, such as a missing column or a bad date format.
Show how to fix the file
List the exact change and link a sample file.
Offer to do it for them
For big accounts, offer to run the import yourself.
Say what happens to partial data
Be clear whether anything was imported and how to avoid duplicates.
Confirm when it works
Follow up once the import succeeds.