One possible solution is to use a plugin called BBP Improvements for Yoast which is a helpful tool to make Yoast wordpress seo support bbpress better. You can install and activate this plugin from your WordPress dashboard and see if that helps.
Another possible solution is to use the excerpt variable in the Admin > SEO > Search Appearance > Content Types (tab) section. There you will have a ‘Topics (topic)’ section where you can add a default meta description using the excerpt variable. This will automatically output the first part of every topic as the meta description.
