You need to qualify the attribute with a list. There are three lists in the server, request, control, reply.
The use of request (default for unqualified attributes) and reply can be inferred from their names. Control (also called 'check' in older modules) holds attributes which affect the behaviour of the modules. Expiration
is a control attribute so you need to qualify expansions involving it with control:
Reply-Message := "Your account has expired on %{control:Expiration} go to ab.com for extending!"