How to access to Gitlab CI variables

I'm trying top create a pipeline for my project and i try to access environment variables in different scenarios:

.gitlab-ci.yml:

variables:
  PROJECT_NAME: ${PROJECT_NAME}
  REPOSITORY_NAME: $REPOSITORY_NAME

prepare:
  stage: prepare
  rules:
    - if: $CI_PIPELINE_SOURCE == "merge_request_event" && $CI_MERGE_REQUEST_TARGET_BRANCH_NAME == 'develop'
      variables:
        ENVIRONMENT: dev
        CUSTOM_ENVIRONMENT_NAME: development
  environment: $CUSTOM_ENVIRONMENT_NAME
  image: docker:stable
  script:
    # Specify in global variables
    - echo $PROJECT_NAME
    - echo ${PROJECT_NAME}
    - echo "$PROJECT_NAME"
    - echo $REPOSITORY_NAME
    - echo ${REPOSITORY_NAME}
    - echo "$REPOSITORY_NAME"
    # Specify in stage variables
    - echo $ENVIRONMENT
    - echo ${ENVIRONMENT}
    - echo "$ENVIRONMENT"
    # Specify in CI custom variables
    - echo $AWS_ECR_REGISTRY
    - echo "$AWS_ECR_REGISTRY"
    - echo ${AWS_ECR_REGISTRY}
    # Specify in CI predefined variables
    - echo $CI_ENVIRONMENT_NAME
    - echo ${CI_ENVIRONMENT_NAME}
    - echo "$CI_ENVIRONMENT_NAME"

The output is:

$ echo $PROJECT_NAME
${PROJECT_NAME}
$ echo ${PROJECT_NAME}
${PROJECT_NAME}
$ echo "$PROJECT_NAME"
${PROJECT_NAME}
$ echo $REPOSITORY_NAME
$REPOSITORY_NAME
$ echo ${REPOSITORY_NAME}
$REPOSITORY_NAME
$ echo "$REPOSITORY_NAME"
$REPOSITORY_NAME
$ echo $ENVIRONMENT
$ echo ${ENVIRONMENT}
$ echo "$ENVIRONMENT"
$ echo $AWS_ECR_REGISTRY
$ echo "$AWS_ECR_REGISTRY"
$ echo ${AWS_ECR_REGISTRY}
$ echo $CI_ENVIRONMENT_NAME
staging
$ echo ${CI_ENVIRONMENT_NAME}
staging
$ echo "$CI_ENVIRONMENT_NAME"
staging

So i don't understand how to access the environment variables in each case.