Go to any dokuwiki page, edit it, while in edit mode click the link "syntax".  An error page appears with the message in the subject.

Here is the error log:

2025-12-19 16:44:07     /usr/share/dokuwiki/inc/Feed/FeedParser.php(11) Error: Class "SimplePie\SimplePie" not found
  #0 /usr/share/dokuwiki/inc/load.php(224): require()
  #1 /usr/share/dokuwiki/inc/load.php(103): class@anonymous->autoloadCoreClass()
  #2 /usr/share/dokuwiki/inc/parser/xhtml.php(1337): class@anonymous->autoload()
  #3 /usr/share/dokuwiki/inc/parserutils.php(691): Doku_Renderer_xhtml->rss()
  #4 /usr/share/dokuwiki/inc/parserutils.php(158): p_render()
  #5 /usr/share/dokuwiki/inc/parserutils.php(97): p_cached_output()
  #6 /usr/share/dokuwiki/inc/Ui/PageView.php(67): p_wiki_xhtml()
  #7 /usr/share/dokuwiki/inc/Action/Show.php(40): dokuwiki\Ui\PageView->show()
  #8 /usr/share/dokuwiki/inc/template.php(120): dokuwiki\Action\Show->tplContent()
  #9 [internal function]: tpl_content_core()
  #10 /usr/share/dokuwiki/inc/Extension/Event.php(134): call_user_func_array()
  #11 /usr/share/dokuwiki/inc/Extension/Event.php(200): dokuwiki\Extension\Event->trigger()
  #12 /usr/share/dokuwiki/inc/template.php(102): dokuwiki\Extension\Event::createAndTrigger()
  #13 /var/lib/dokuwiki/lib/tpl/dokuwiki/main.php(59): tpl_content()
  #14 /usr/share/dokuwiki/inc/actions.php(30): include('...')
  #15 /usr/share/dokuwiki/doku.php(131): act_dispatch()
  #16 {main}