I had documented my problem in the attached 3-page PDF file which includes all the relevant screenshots and procedural details.
I'd be grateful for any useful advice, tips or suggestions.
Many thanks.
Check all your other fields that you are not importing and make sure they are not set to "required"
Have null allowed and dont have a unique index on them.
The error message is clear enough. It didn't like what you were doing with a keyed (i.e. has index) field. You might need to compact & repair the database just before you perform the Append because that forces re-evaluation of everything including indexes. It also resets autonumber fields to the correct "next value" and assures that an appropriate value can be derived for such fields.