Consider below code :

parameters:
  - name: containerRegistry
    displayName: Azure Container Registry Service Connection
    type: string
    default: 'acr-amaest'
    values:
      - acr-amaest

  - name: deploymentSteps
    type: stepList
    default:
      - script: echo "Add your deployment steps to your template"
        displayName: Empty deployment step

  - name: branchMapping
    type: object
    default:
      develop: build
      staging: staging
      main: release

variables:
  - name: environment
    ${{ each branch in parameters.branchMapping }}:
      ${{ if eq(variables['Build.SourceBranch'], 'refs/heads/${{ branch.Key }}') }}:
        value: ${{ branch.Value }}

stages:
- stage: Test
  displayName: ???? Test
  jobs:
    - job:
      steps:
        # Printing empty string
        - bash: |
            echo ${{ variables['environment'] }}
        # Works, prints everything
        - ${{ each branch in parameters.branchMapping }}:
          - bash: echo '${{ branch.Key }} -> ${{ branch.Value }} against ${{ variables['Build.SourceBranch'] }}'
        # Prints nothing    
        - ${{ each branch in parameters.branchMapping }}:
           - ${{ if eq(variables['Build.SourceBranchName'], '/${{ branch.Key }}') }}:
             - bash: echo "Coucou ${{ branch.Key }}"

Could you please help ?