The problem is that this feature is only for premium and ultimate users as you can see here at the top

See how it looks

https://docs.gitlab.com/ee/ci/secrets/#use-vault-secrets-in-a-ci-job

Although I don't know why, but they left Free on this page, which made me confused too.

See how it looks https://docs.gitlab.com/ee/ci/secrets/

PS. Now in 17.+ versions you are not allowed to use CI_JOB_JWT, instead use id's https://docs.gitlab.com/ee/ci/yaml/index.html#id_tokens