Thank you for reaching out to the Community. In QuickBooks Self-Employed, we're unable to change the currency. Once you're using the QBSE US version, $ is your home currency by default. Also, we don't offer QBSE in South- Africa. Let's double-check the QBSE country you've selected.
Additionally, the multi-currency feature is only available in the version of QuickBooks Online. You may want to check out these articles to learn more about how multi-currency works in QuickBooks Online: