pnpm not found

Hi! I'm having trouble building my app, and haven't had success with any of the suggestions with other "pnpm not found" searches.
My railway.json:

{
  "$schema": "https://railway.app/railway.schema.json",
  "build": {
    "builder": "NIXPACKS",
    "nixpacksPlan": {
      "providers": ["python"],
      "phases": {
        "setup": {
          "nixPkgs": ["..."]
        }
      }
    },
    "buildCommand": "pnpm i --frozen-lockfile --force && pnpm -F frontend build"
  },
  "deploy": {
    "startCommand": "python manage.py migrate && python manage.py collectstatic --no-input && gunicorn faves_project.wsgi",
    "restartPolicyType": "ON_FAILURE",
    "restartPolicyMaxRetries": 10
  }
}

The build error I'm seeing:

#12 [stage-0 8/10] RUN pnpm i --frozen-lockfile --force && pnpm -F frontend build
#12 0.295 /bin/bash: line 1: pnpm: command not found
#12 ERROR: process "/bin/bash -ol pipefail -c pnpm i --frozen-lockfile --force && pnpm -F frontend build" did not complete successfully: exit code: 127
-----
> [stage-0 8/10] RUN pnpm i --frozen-lockfile --force && pnpm -F frontend build:
0.295 /bin/bash: line 1: pnpm: command not found
-----
Dockerfile:24
-------------------
22 | # build phase
23 | COPY . /app/.
24 | >>> RUN pnpm i --frozen-lockfile --force && pnpm -F frontend build
25 |
26 |
-------------------
ERROR: failed to solve: process "/bin/bash -ol pipefail -c pnpm i --frozen-lockfile --force && pnpm -F frontend build" did not compl

I have a pnpm-lock.yaml in my root directory.
My project id: 90e8bbd5-6381-4906-9853-3d5ea1445654