When the Sum function is used in a text box expression, it
operates on a field in the form's dataset (the form's Record
Source table/query).  It works for any level and any view of
a form (or report).  Normally, it's used in the form's
Header or Footer section.  By its very nature, it does not
work in the Page Header or Footer.

I suspect that your problem is that you could be trying to
use it to sum a form control's value instead of a
table/query field. Either that, or that you spelled the name
of the field incorrectly.

On the other hand, DSum is a VBA function that goes directly
to a table or query to sum a field in the entire table,
subject to a filtering expression.  While it is generally
slower than Sum, it is usually used for a different purpose.