Try the following in order to resolve the issue.

Open the spreadsheet or import file and confirm that the field already exists in Altru (Lookup ID, Bill to, etc):
  1. Example for Lookup ID: To confirm, go to Constituents > Constituent search and search by the Lookup ID field

Change the Search list fields configuration:
  1. Go to Administration > Import
  2. Highlight the import process, and click Edit
  3. Click on the Options tab
  4. Under Search list fields, select either Quick find or ID (whichever setting it was NOT on before)
  5. Click Save
  6. Click Start import to run the process again