that post content is wrapped inside a <div> with the ID of ftwp-postcontent – presumably this is being added by a Plugin is probably what WP cannot extract the excerpt.
Also your first element in the content is a GenerateBlocks container which i can see no use for – you should remove that.
The default Marketer is only using Recent Posts Widget With Thumbnails and
Social Icons Widget & Block by WPZOOM so try disabling all others you are using.