I just had the same issue! The problem was that the lock files were inside the monorepo folders - they must be only in the root folder, as described in the documentation: https://opennext.js.org/aws/common_issues#cannot-find-module-next

Make sure you have a single lock file (package-lock.json, yarn.lock, or pnpm-lock.yaml) in the root of your project, not in the individual app directories within your monorepo.