Originally Posted by
christep
Computer says no because the ticket you have is, apparently, a DONE3 and SEZ is in a 4th continent.
You should be able to pay the historical fare difference to upgrade from DONE3 to DONE4.
It could be possible that the ticket is purchased before the rule change, when the stop in Europe needed to be transit without stopover so that the computer doesn't allow to add Africa anymore. Even if that's the case, you might be able to get over it by referring to current rules.