how can i let search pages be indexable?

Hello,
Yoast SEO makes sure your search pages are set to noindex. This is not a setting, this is simply how it should be done according to the Google Guidelines. So we won’t be adding a feature that’ll make this editable in either Yoast SEO or Yoast SEO Premium.
However, we do have an option for developers to change this. You can find more information here: https://kb.yoast.com/kb/noindex-for-special-pages-in-yoast-seo/

thanks a lot for the fast answer!