It seems pnpm 11 is not supported. I wrote an issuue here, but maybe this not a bug but more of a feature request:
https://github.com/vercel/vercel/issues/17434
The problem is that many projects ahould and will upgrade to pnpm 11 and 12 now and cannot wait for the vercel CLI to make it work with that package manager.
Is there a workaround for this? Corepack is not a good solution as it is being removed from Node.js 26.