Pipeline job not starting on merge request

Hi Guys,
I am new to Gitlab CICD and still learning.
It is personal free account
I have created simple project to try pipelines with jobs are run only on merge request. I have tried with following:

script: “:”
only: [‘merge-requests’]

But when I create merge request pipeline does not start and all merge checks pass.

When I remove “only: [‘merge-requests’]” part and run pipeline it runs without any issue.
Do you have any idea what I am doing wrong.
I have also tried with
- if: $CI_PIPELINE_SOURCE == “merge_request_event”
But it does not work either

Thank you in advance