You are doing nothing wrong. The copy and paste script steps, from, oh maybe version 1? have only worked if the field referenced in the script step is present and accessible on the current layout. Here are several options you can consider:
a) Put a copy of this field "off the edge" of the layout (in the grey area).
b) Have the script that does the copy change layouts to a layout where the field is accessible, copy the data and then change back to the current layout. The user should not notice that the layout ever changed.
c) Open a new window, (which can have negative coordinates specified to keep it from being visibile to the user, change layouts in this new window, copy the data and then close the window...
d) Hide an editable copy of the field by specifying True in the Hide Object When setting for the field. I think this will work without having to make the field visible, but I haven't tested this to see for sure.
Thanks Phil, since (d) was untested, decided to give that a try. It does work providing another script line is added to redirect to another field or object. I used "go to field" to redirect back to the Title field which worked like a charm and, as this highlights the field, shows the user something happened.