Try each of the following steps in order until the error/issue is resolved:

1. If importing new attributes, ensure that the import parameters are set to import new records
2. On the Map Fields tab of the import parameters, make sure that the fields are mapped according to how the attribute will appear in the constituent's record and not how it appears in Configuration, Attributes
3. Change the import file to use constituent attribute headers. For more information on importing constituent attributes, refer to BB8670