@phorge Oh, after looking more deeply, GitLab has two types of issues. One is "issue" and the other is "work item" (or "task"). And only "work items" can be assigned as a child task. A "work item" can be created by the "New Task" button in the "Add" dropdown. Very confusingly, a "work item" can be converted into an "issue," but only if it does not have a parent. However, the only way to create a "work item" is to create it with a parent, using the steps I described before. There are two routes to display an issue, namely /work_items/:id and /issues/:id, but they are almost the same, and only the former route has the UI to convert between an "issue" and a "work item".