I know this says 'solved', but I'd like to chip in my 2 kobos nonetheless.
First, as others have said, it's in line with Upwork's rules for people to be able to specify which countries they want to get applications from and which ones they don't want to. I don't have a problem with it - there are legitimate instances where it'll be better to have writers from a particular location.
Since this one apparently doesn't mind everyone else but Africans, the post was clearly prejudiced but not necessarily racist (although that's likely to be present too, based on real-life experience).
I agree with you that there are a lot of high-quality writers from Africa (whether native or migrants) and I understand why the posting would be offensive. As someone who has been published in a few major publications, it irritates me to see such illogical discrimination too.
Sometimes, when I come across (on other job boards - I'm quite new on Upwork) similar "native speakers only" guidelines but I really want the job, I apply anyway and SHOW that though I'm not a native-English speaker (this is another absurdity but for another day), I'd be the best for the job based on my qualifications and experience. It often works. You might want to try it too if you come across a very attractive job, not a poor one like this.
