In the outputs example usage, this is the most likely deprecated GitHub Actions syntax:

- name: feedback
        if: steps.automerge.outputs.mergeResult == "merged"
        run: |
          echo "Pull request ${{ steps.automerge.outputs.pullRequestNumber }} merged!"

Whereas working conditional code uses yaml curly brackets:

- name: feedback
        if: ${{ steps.automerge.outputs.mergeResult == 'merged' }}
        run: |
          echo "Pull request ${{ steps.automerge.outputs.pullRequestNumber }} merged!"

Full error

Invalid workflow file: .github/workflows/automerge.yml#L48
The workflow is not valid. .github/workflows/automerge.yml (Line: 48, Col: 13): Unexpected symbol: '"merged"'. Located at position 40 within expression: steps.automerge.outputs.mergeResult == "merged"