In one of your pages you attempted to access an environment value that is not provided in the environment.
When accessing environment variables on the client they must be prefixed with NEXT_PUBLIC_ to signify they are safe to be inlined for the client.
When accessing environment variables on the server in getStaticProps, getServerSideProps, or an API route the value must be provided in the environment at runtime.
Either remove the code accessing the env value, populate it in your .env file, or manually populate it in your environment before running next dev or next build.