Tags do not live in a Perspective Session, so they may not invoke (most) Perspective package functions. If you look in your logs, you will see failed usages for each time that script executes.
Why are you sending this as a message instead of just binding some property within the session against the Tag Value? Configure a custom session property, then bind that prop against your Tag. Place a change script on the property. Be aware though that session property change scripts may not directly open Popups - Session property change scripts which attempt to do this MUST specify a pageId kwarg to the openPopup function.