It sounds like your form is not bound to a recordsource, and you are trying
to refer to fields from different potential recordsources directly in
textboxes.  That's not the way it works, and doing so will give #Name
errors, because there is no field in the forms recordsource with the names
you have entered.  Although it's possible to work with unbound forms, it's
not usual - and it generally involves considerable amounts of VBA code
behind the scenes.  Most often, a form is bound to either a table or a
query, and then the fields from that table/query can be accessed via
controls on the form.

For what you want, include both Field1 and Field2 in your query, and set the
recordsource of the form to the query (enter the name of the query, or
select it via the dropdown in the property box).  Then you can set the
control sources for your textboxes to Field1, Field2 and Result (the
calculated field in your query), again either by typing the field name into
the box on the property sheet, or via the dropdown.  Note that you do not
need an = sign before the field name.

It's also possible to do calculations on the form, by setting the
controlsource of a textbox to an expression.  So you could have the form
bound to the table, with textboxes bound to Field1 and Field2, and in an
unbound textbox enter "=[Field1] + [Field2]" as the controlsource for that
textbox.  Another common trap, particularly if you set up your form via a
wizard or drag-and-drop fields, is to have a textbox with the same name as a
field in the form's recordsource; if you enter an expression referring to a
field with the same name as a textbox, you will also get a #Name error.  For
this reason, it's good practice to use a naming convention for all your
controls to avoid them having the same name as an underlying field (eg. name
a textbox as txtField1, rather than using the default of Field1).

HTH,

Rob