Unable to access CI/CD variables in pipeline

Hello,

I’m exploring gitlab CI/CD & facing an issue to access variables set in UI. Can someone please help me out?

default:
  image: alpine

variables:
  EXPO_USERNAME: $EXPO_USERNAME

stages: # List of stages for jobs, and their order of execution
  - build

build-job: # This job runs in the build stage, which runs first.
  stage: build
  script:
    - echo $EXPO_USERNAME