It could be that the CHMOD settings for your /modules directory is incorrect. Ensure that both /modules and /modules/payfast both are CHMOD 0755. If it is 0777 the PrestaShop installation won't see the module and reports an error 404, page not found, or, throws an internal server error.
On PrestaShop, why isn’t my payments being marked as paid after a successful transaction?
or drag and drop files