I’ve run in to this same issue (late 2024). The workaround I’ve found that works is to create the manifest from the “Orders” page (and NOT the “Shipping labels” page). On the default Orders page, click the “More actions” dropdown button, and choose “Create manifest” from the available options. This correctly launches the shipping-manifests app.
Trying to use the “Pickups and Manifests > Create manifests” button on the “Shipping labels” page does not work (as many people in this thread have documented). YMMV.