The "Build Directory Not Writeable" error usually occurs when the file system does not permit writing to the designated directory. A common scenario for this error is when you initiate a [custom server](/docs/pages/building-your-application/configuring/custom-server) in development mode on a production server.
These production servers often disallow writing to the filesystem after your application is built, causing this error.
If you're deploying a custom server with a server file (let's assume it's named `server.js`), you should modify the scripts key in your `package.json` to the following:
- [Custom Server documentation + examples](/docs/pages/building-your-application/configuring/custom-server) - Learn more about how to effectively set up and manage an ejected server in Next.js.