This error means the build expects a secret named API but Railway isn't passing one. Railway exposes your service variables as build secrets, so the secret id must exactly match a variable name on the service. Check these in order: Dockerfile secret mount — if your Dockerfile has RUN --mount=type=secret,id=API ..., you must have a variable named exactly API (case-sensitive) in your service → Variables tab. Either add it, or rename the mount id to match your actual variable (e.g. API_KEY). Railpack picking up a stray reference — if you're not using a Dockerfile, Railpack may have detected ${API} somewhere (config files, application properties, build scripts) and turned it into a required build secret. Check your build logs for the generated railpack-plan.json section (just before "load build definition") — if API is listed under secrets there, find and fix the reference or add the variable. Variable added but not redeployed — variables added after the last deploy aren't picked up automatically at build time. After adding API, trigger a fresh deployment (Cmd/Ctrl+K → "Deploy Latest Commit"). If none of that matches, share your Dockerfile (or confirm you're on Railpack/Nixpacks) and the few lines above the error in the build logs.