Preview tools are built for speed. Production hosts are built for repeatable builds, safe runtime behaviour, correct routes, real domains and controlled secrets. A deployment failure often appears when generated code crosses that boundary.
The issue may be a missing environment variable, a server-only call placed in browser code, an API URL that still points at localhost, a route that needs host rewrites, or a dependency that preview tolerated but the production build rejects.
That does not mean the app was a mistake. It means the production layer needs the same care as the build layer.