What should I do if I want the message box to return a message if the value is null?...

Good morning. I have two tables (Batches) and (Archive). (Batches) is a linked table that contains all of the existing data and the other (Archive) I am keying data into, through a form. I want to be able to key into a form that writes to (Archive), but before it saves or moves to the next field, I want to make sure that the data already exists in (Batches) and prompt the user if it doesn't. The reason being is that I don't want to save any...