Invalid Data Type in Field

I'm trying to import a list containing two columns: unique redemption code and email address. I keep getting import failure reason of Invalid data type in my redemption code field. It is a mix of letters and numbers about 9 characters long. I've tried both String and Text as the custom data field type with no luck. What am I doing wrong?

The goal is to send an email to each of these displaying their unique redemption code.