You need doctrine/annotations for route annotation support.

Nyholm · Nyholm · commit f5e59ea1fd22 · 2020-10-13T10:55:37.000+02:00
diff --git a/routing.rst b/routing.rst
@@ -33,7 +33,7 @@ once in your application to enable them:
 
 .. code-block:: terminal
 
-    $ composer require annotations
+    $ composer require doctrine/annotations
 
 .. versionadded:: 5.2
 
