Try each of the following until the issue is resolved.
  1. Ensure the data type for the attribute matches to the values being imported for the attribute description.  If an attribute with a data type of table is being imported, the description value must be an existing table entry. If it does not yet exist, add the table entry in Configuration.
  2. Ensure the extension numbers are set correctly in the import parameters on the Fields tab for each attribute that is being imported. If importing only one attribute, the extension should be the same for all attribute fields. If importing multiple attributes, the extension for the first set of attributes fields should be 00. The extension for the second set of attribute fields should be 01.
For more information, see How to import constituent attributes (BB8670) or How to import multiple attributes (BB129253).