The OP 楊筱誼 confirms in the comments:
"Setting default template" is only available if you have a Starter (or higher) license for a self-managed installation of GitLab or Bronze for GitLab.com.
My trial license seems to have expired, so it cannot be displayed.
I can not find text area.
Precisely because, as described in the official documentation "Description templates", you do not have yet created a .md
file in the .gitlab/merge_request_templates/
directory of your repository.
Once you have created, committed and push that file, then you will see that field:

I do not want to use template(.gitlab/merge_request_templates/).
Then... you won't see that field.
Don't forget the visibility:
The visibility of issues and/or merge requests should be set to either “Everyone with access
” or “Only Project Members
” in your project’s Settings / Visibility, project features, permissions section
, otherwise the template text areas won’t show.