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
• "Lookup" fields that have a specified Key Field for which the Prohibit duplicate values option has not been enabled
• Fields specified in the Field Mappings section in 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 the one that does not accept bulk adding or updating via file import, refer to Import/Export of App Data.

• If a field is specified as Key Field for a "Lookup" field, you cannot import data from a file for that field unless its Prohibit duplicate values option is enabled.
  Check the field setting in the source app, and enable the Prohibit duplicate values option if it is disabled.

• The fields specified for Field Mappings of a "Lookup" are not included in the target of file import when you import data from a file. If you import data to the "Lookup" fields, 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 the Permissions for Fields setting in App Settings.
If necessary, grant the view or edit permission for relevant fields to the user who will carry out the Import from File operation.
Configuring Permissions for Fields