Why are Wordpress Sitelinks not Appearing in Serp?

I would contact support...

https://wordpress.org/plugins/wordpress-seo/

If the structure of your site doesn't allow algorithms to find good Sitelinks, or if the Sitelinks for your site are not relevant for the user's query, they won't be shown.

If the structure of your site doesn't allow our algorithms to find good Sitelinks, or if the Sitelinks for your site are not relevant for the user's query, they won't be shown.

Sitelinks are automatically created by Google. You don’t have any control on preferring one landing page for Sitelink. Instead, you can remove a specific URL if you don’t want them to be shown in the search engine as a Sitelink.

Try this:

• On the Search Console Home page, click the site you want.
• Under Search Appearance, click Sitelinks.
• In the 'for this search result box' complete the URL for which you don't want a specific sitelink URL to appear.

Let me know if this helps.

Regards,
Caleb

Google only determines if sitelinks will emerge in SERP or not. Its not up to SEO only that sitelinks will appear in SERP. Your website should have high traffic and popular.