Is this a riddle?
As a mark is not set up for the purpose of missing the aim, so neither does the nature of evil exist in the world.
Targets are not put up with the intention of the athlete missing them. Misses occur due to human shortcomings, not because of intention.
The unstated presumption of the second part of the sentence is that God, who is perfect, created the world. With this is mind, the idea that evil exists would mean that either God intended it, or that God is imperfect. Both of those ideas are implausible to Epictetus.
That's all very nice, but what does it have to do with Stoicism? I think the point is that, if true evil does not exist, we don't need to fear it. What we see as evil is just the shortcomings of other people; there is no grand plan to keep us down. This is more important if, as the Stoics did, you believe in fate.