fix issue4333

CHANGES.rst

@@ -24,6 +24,8 @@ Released 2021-10-04
 -   Fix the order of ``before_request`` and other callbacks that trigger
     before the view returns. They are called from the app down to the
     closest nested blueprint. :issue:`4229`
+-   Fix ``after_this_request`` fails with a meaningless error when used
+    outside request context. :issue:`4333`
 
 
 Version 2.0.1

src/flask/ctx.py

@@ -130,7 +130,14 @@ def after_this_request(f: AfterRequestCallable) -> AfterRequestCallable:
 
     .. versionadded:: 0.9
     """
-    _request_ctx_stack.top._after_request_functions.append(f)
+    try:
+        _request_ctx_stack.top._after_request_functions.append(f)
+    except Exception:
+        print(
+            "This decorator can only be used at local scopes "
+            "when a request context is on the stack.  For instance within "
+            "view functions."
+        )
     return f