I don’t know if you understood.
I am not trying to add a role that isn’t on the subsite already. In fact, I am only trying to change to a default WordPress role.
So as a Super Admin I go into Network Admin, Sites and edit a subsite, go to users and try to change a user from Author to Editor then I get the error.
If I go into that subsites admin page, then users and change it there it works.