You can import values into "Lookup" fields via file import.
Using the imported values as the lookup key, "Lookup" fields retrieve data from other apps.

Note that, in order for values to be imported into a "Lookup" field via file import, the conditions below must be met in advance by the app that contains the data to be retrieved (the datasource app).

• The Prohibit duplicate values checkbox is selected in the settings of the field specified as the Key Field of the "Lookup" field (This condition must be met only when the classic file import method is selected to import a file)
• In the Permissions for App setting, the user who carries out the Import from File operation is granted the permission to view records.

The following describes the ways to check whether these conditions are met.

• How to check the field specified as the Key Field and the app to which it belongs
  Open the app into which you want to import your file, open the field settings of the "Lookup" field, and check the Datasource App and Key Field.
  If the message "No permission" appears on the "Lookup" field and you cannot open its field settings, it means that you do not have permission to view records for the app from which data is to be retrieved (the datasource app).

• How to check whether the prohibition of duplicate values is enabled for the field set as the Key Field
  Open the app that contains the data to be retrieved (the datasource app), open the settings screen of the field set as the "Lookup" field's Key Field, and check whether the Prohibit duplicate values check box is selected.
  If the check box is not selected, select it and save. Then, update the app to apply the setting change to the app.
  

• How to check whether you have permission to view records for the app from which data is to be retrieved
  Open the app that contains data to be retrieved (the datasource app) and check if the error message "No privilege to proceed" appears.
  If the error message appears, it means that you don't have the permission to view the app's records.
  If a user without the permission carries out the Import from File operation, the "Lookup" field fails to retrieve data from the app and the Import from File operation results in an error. Either grant the permission to the user who carries out the Import from File operation (Permissions for App) or ask the user with the permission to carry out the operation.
  Configuring Permissions for Apps

If you cannot import values into "Lookup" fields via file import even though the conditions mentioned above are met, it may be due to the app you are importing your file into or the file you are using for the import.
Refer to the following pages and check the app and the file.

• Checking the Destination App for File Import
• Supported Formats for Adding/Updating Records (Lookup)
• What to Do If Adding/Updating Records in Bulk Failed