Thank you for your post.
Take the file down from the host and open the file locally with FileMaker Pro 5/6.
Do you have any startup script? If so, check to see if "tax code" is a field or a file.
Check your field definitions and see if Tax Code is present. If not, check for fields that include Lookups into other files.
Ultimately, whether this is a file or a field, your best option will be to restore from a back up copy if you have one.