In Cubase you have to create an additional Output for the DI Track.
Using my Focusrite 18i20 I Adresse in Cubase Line 9 or SPDIF L for this Output
In Focusrite Software, I have to Adresse DAW9 to SPDIF L.
May be that you have touse you have 7 instead of 9 my case, because you are using a Focusrite 6i6