Is it possible to conditionally get a variable group using a variable?

I have a Variable group called "CI environments" with those variables:

• CI_Branch1: Database1
• CI_Branch2: Database2

I have a yaml pipeline with a variable called CIDatabase in a branch called Branch1

Can I set the value of CIDatabse like this?

echo "##vso[task.setvariable variable=CIDatabase ]variables.$(TR_CI_${{variables['Build.SourceBranchName']}})"

Build.SourceBranchName is a predefined variable available in templates, so you don't need to set output variables.

Try using template expression syntax ${{ ... }} instead:

variables:
  - group: 'CI environments'

  - name: CIDatabase
    # e.g. if source branch name is 'dev' then variable value will be '$(CI_dev)'
    value: "$(CI_${{ variables['Build.SourceBranchName'] }})"

steps:
  - script: |
      echo "Database: $(CIDatabase)"