Hi 21765e022a14 - Jira's issue hierarchy is designed to enforce the order in which work is broken down. As part of this, subtasks sit under L0 issue types. There was a bug previously where subtasks could be created under L1+ issue types. I can suggest a couple of things here:

1. Create an issue type named "subtask" at the level below the parent you are trying to breakdown
2. Demote the parent issue type to the L0 level and then you will be able to create subtasks

Also, if the parent issue types already had subtasks prior to the change, the + button will still appear in the subtask panel further down the issue and subtasks can be created here

Also, if you want to change any existing issues to a different type, for example if you wanted to migrate existing subtasks to another L1 issue type, bulk edit is the fastest way to do this (this can be done from the backlog or via the "issues" view then three dot menu)

hope this helps!

For us, the only way to create a subtask is by using the "Create subtask" button. When it was switched to "Create a child", we were not able to create subtasks anymore. This means 10 out of 10 teams couldn't breakdown their work anymore. We really need that button