Unable to parse YAML: mapping values are not allowed here
You simply have an incorrect yaml syntax in your workflows jobs list.

From what I could very quickly understand on circleCI's documentation, jobs names are accepted as a shorthand if you don't define any parameters. If you have parameters, the list element is a hashmap with top element being the job name. In your specific case, this should give (note the column after the names of jobs with params):

workflows:
  version: 2
  minimal:
    jobs:
      - build
      - tests:
          requires:
            - build
      - release:
          requires:
            - tests
          filters:
            tags:
              only: /^v.*$/