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After accessing "Execution History" of a scheduled issue with such status, you will be able to spot that "incomplete" is only relevant to deleted issues - it doesn't affect creation of new issues, linked to a still existing ones in Jira. In other words - it's just a warning that it's impossible to create linking with an issue that is not present in Jira any longer (after execution of such, linked issue won't be present in newly created issue).
As for the sub-tasks - any issue placed in "Scheduled Issues" table (also with "inactive" status), can be managed with sub-task linking. To do do, simply click "Add subtasks" button placed next to name of SI.
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A new prompt with all necessary information will be presented:
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After clicking any of the two present "Add Subtask" buttons, next screen will load, with a linking option present. On a top of it you must provide a "subtask name", then scroll down and fill-in below presented information, as in the below presented example:
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After pressing "Submit" (4↗) new linked sub-task will be created and "No subtasks created" (1↖) information will disappear.
Before pressing "Submit" button, select a reason for linking issue (2↗) i.e. "relates to" and then find one more or more that fits to it.
After submitting recently edited one, "Add Subtask" (3↖) button will become active again, allowing you to create more dedicated subtasks (up to 15 max.), each with unique name, linked issues and fields to fill in.
For more of the features in regards to instant operations on existing sub-tasks in regards to cloning and removing it, please refer to below image:
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Any created sub-task can be quickly replicated or removed - when you hover a mouse cursor over any of existing sub-tasks(1↖), two more options are to appear at the same time - clone and delete. Move cursor horizontally to the right (stay on the same line as a sub-task is placed) to choose a desired option.