I'm sorry to hear that you're receiving an error when you're trying to import a file of contacts. I'm happy to help.
Here are the steps on how to format your file for importing to get you started.
As for the file, it's hard for me to give next steps without the file, so if you could, could you email the file to social_support(at)constantcontact(dot)com along with your username and referencing this post in the Community. Have you tried saving it as an Excel Workbook instead of a CSV to see if that allows for import? Also if you have any special characters, could you try to remove those before importing?
Looking forward to working with you!
For immediate assistance, you can contact our Support Team here.