What's the Control Source of the form textbox? And what's the
Recordsource of the form?

#NAME? means that Access doesn't recognize the fieldname as being a
field within the Form's recordsource query.

Do note that it's almost never appropriate to store calculated values
in a Table. Storing derived data such as this in your table
accomplishes three things: it wastes disk space; it wastes time
(almost any calculation will be MUCH faster than a disk fetch); and
most importantly, it risks data corruption. If one of the
underlying fields is subsequently edited, you will have data
in your table WHICH IS WRONG, and no automatic way to detect
that fact.

Just redo the calculation whenever you need it, either as a
calculated field in a Query or just as you're now doing it -
in the control source of a Form or a Report textbox.

                 John W. Vinson[MVP]