Shopify Payments is currently only available in 17~ selected countries and regions, and is not available in any African nations at this time. The gateway has become eligible to merchants in more countries over time and may continue to expand to other countries in the future, but at this time no imminent expansion of availability is due. Our development team ask that we log requests from merchants in countries who are requesting to use Shopify Payments in order to help gauge interest, and I will be sure to pass your feedback onto them.
In the meantime, you can use other third-party gateways that integrate with Shopify to accept payments from your customers. If you head to the Africa tab here and select your country, you'll be presented with a list of gateways you can use on your Shopify store.