react-day-picker is behind the shadcn Calendar component v0 added. Version 8 declares support up to React 18, and npm refuses to install it next to React 19.
Proper fix: upgrade it. v9 supports React 19, and the current shadcn calendar is written for v9:
npm install react-day-picker@latest
npx shadcn@latest add calendar --overwriteQuick unblock if you have a deadline: an .npmrc in the repo root with
legacy-peer-deps=true
Vercel reads that file too. It works because v8 runs fine on React 19 in practice, but you're hiding the warning, not fixing it.