get_the_post_thumbnail_url does not return anything if image size is set

I want to display the post-thumbnail image in the header using get_the_post_thumbnail_url() with defined size, however if I am using it without specifying the size, like get_the_post_thumbnail_url() it is displaying the thumbnail img, but if I am using it like: get_the_post_thumbnail_url('thumbnail') nothing is shown.

How to Get The Excerpt of the page that displays Blog Posts

I use a static page (Blog) to display the latest posts. I want to show the excerpt of that page next to the title. However, wen I use wp_kses_post( get_the_excerpt() ), I get the excerpt of the latest post from the post archive loop. I also tried with wp_kses_post( get_the_archive_description() ) but it also does not seem to work.. Any idea how this can be done?