I suspect you are using "create issue".
You can't create subtasks from there, you need to go to the issue that you want to make them a sub-task of. Use the tools menu to create one, then the section will appear and offer you the + option to add further ones.
No, not "create", you need to use "create sub task", which is under the tools menu on the issue (far right hand side of the issue view screen)
You can't edit the issue type to convert it to a sub-task either, and there are no other issue types in your project that are compatible, so you can't "edit" the type at all.
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