Hi,
South Africa (and many, many other countries) have been blocked from registering due to extremely high levels of abuse.
In order to keep this service free, and also functioning well for all our users, we have chosen to only whitelist certain countries from which abuse levels are low.
Unforunately, at this time there is no way to bypass this restriction. However, we do revisit the whitelist from time to time, so it's possible that South Africa will be added at some point.
Thanks,