The filesystem does not allow writing to the specified directory. A common cause for this error is starting a [custom server](https://nextjs.org/docs/advanced-features/custom-server) in development mode on a production server, for example, [Vercel](https://vercel.com) which [doesn't allow you to write to the filesystem after your app is built](https://vercel.com/docs/deployment-types/node#file-system-specifications).