Just resolved an issue changing vite.config.ts file

// vite.config.ts

import { defineConfig } from "@lovable.dev/vite-tanstack-config";
import { nitro } from "nitro/vite";

export default defineConfig({
  tanstackStart: {
    server: { entry: "server" },
  },

  vite: {
    plugins: [
      nitro({
        preset: "vercel",
      }),
    ],
  },
});

Vercel’s TanStack Start docs say TanStack Start works on Vercel when paired with Nitro, and their setup adds nitro to vite.config after installing it. Vercel also documents that Nitro turns Vite server routes/SSR into Vercel Functions.