Try each step in order until the issue is resolved:

1.  In the Import parameters, select the Fields tab.
2.  Click Clear Fields.
3.  Click Update Fields.
4.  Re-map any fields do not match up to a corresponding Raisers Edge field.
5.  Add a unique extension to any field that is being imported more than once.
6.  Double-check the mapped Raisers Edge fields to ensure a field is not mapped incorrectly.
7.  Check the import type is being used.  For example, if importing constituent attributes, a Constituent Attribute import requires each attribute to be on a separate row, but a Constituent import will allow multiple attributes on one row.