Assuming you have activated the WPSSO FAQ Manager add-on, you must first create at least one question & answer under the WordPress admin FAQs > All Questions page, then assign these question(s) to at least one FAQ category.
If the SSO > FAQ Settings > Disable FAQ and Question Page URLs option is unchecked (default), then you can simply use the FAQ category archive page URL to view your questions & answers, and this archive page will include the correct Schema FAQPage markup for Google.
If you prefer NOT to use the FAQ category archive page URLs, you may enable the SSO > FAQ Settings > Disable FAQ and Question Page URLs option, and use the FAQ shortcode instead. You can find the FAQ shortcode in the FAQs > FAQ Categories page.
If you are using the FAQ shortcode, keep in mind that you are adding a FAQPage within another Schema Type for the content. For example, if you include a FAQ shortcode in an article – the Schema Type in the Document SSO metabox will be Article and the Article markup will include the FAQPage (both will be included – in relation to each other – in the Schema graph markup).
If you are using the FAQ shortcode in your content, do not choose FAQPage as the Schema Type in the Document SSO metabox – this will result in an error as adding a FAQPage within a FAQPage is not a valid combination.