There has to be inventory in the SU bucket (and you are inside your window), just like there has to RU inventory to get upgraded to F. While there is generally SU inventory if there are open W seats, there are no guarantees. Whether or not there is SU inventory here is the question. There was a longtime bug where delta.com was always showing F upgrades available (RU bucket) even though there was no inventory. While they have now apparently fixed this, there might still be issues with showing SU availability.
I think many people are unaware that C+ upgrade inventory is capacity controlled. (Just like First Class upgrade inventory). It doesn't happen often, but it does happen.