I am having the same problem on my old Mac Pro, can’t go higher than Mojave.
Glad I still have a newer MacBook Pro to do the update. I probably would have asked
a friend to help me out if I did not have my laptop.
Not sure how many OXI ONE users still use Mojave, I guess only very few, so I understand
OXI to not support it. I can personally live with it.
Hope you have a friend that can help out 😉