Hello, I have tested the Yoaast SEO plugin and it works as expected, so I am guessing its specific to your wp instance. First, please follow the steps mentioned here and see if this resolves it:
If you continue to face the issue after following these steps, please respond here and I will create a private chat for you to provide details. Please note that you will need to have followed the steps and we can only test your site if the steps above are left in place for us to test.