I get the message "The expression is not valid."
You attempted to type an expression that includes the name of a field that is not in the data definition of the page. Any field that you reference in an expression on a data access page must be in that page's data definition. To add a field to the page's data definition, drag the field from the field list to the data access page. If you don't want to show the values in the field on the page, you can hide or delete the control bound to the field. However, even if you delete the control, a field referenced in an expression remains part of the page's data definition.