The following fields are not displayed in the Field in app column on the Import from file screen.

• Fields that do not accept bulk adding/updating via file import
• Fields specified in the Field mappings section of the settings of a Lookup field
• Fields for which you do not have edit permission

If a field is not displayed in the Field in app column, check the following.

Data of some fields (such as Attachment fields) cannot be bulk added or updated via file import.
To check whether the field that is not displayed in the column is one that does not accept bulk adding or updating via file import, refer to the following page.
Import/export of app data

The fields specified as Field mappings of a Lookup field are not included when you import data from a file. If you import data to a Lookup field, the data is automatically retrieved from the associated app.

Reference: Is it possible to import values from a file into a "Lookup" field?

As an app administrator, check Permissions for fields from the App settings screen.
If necessary, grant the view or edit permission for the relevant fields to the user who will carry out the Import from file operation.
Configuring permissions for fields