In OpenCart, PayFast isn’t listed in the Checkout.
Ensure that you are using the latest version of our plugin for the correlating version of OpenCart.
Ensure that you have added a South African geo-zone (at System > Localisation > Geo Zones).
Edit the tax options to include the geo-zone (at System > Localisation > Taxes > Tax Rates). Click 'Add New', and create South African VAT, currently 15%. Be sure to set the Geo Zone setting to the one you created in the previous step.