At the moment is not possible to create a parent-child relationship using CSV for team-managed projects.

It returns the following error:

• Cannot add value [ Issue ] to CustomField [MTTFC-28] in Parent Link with summary 'Task two'. Probably value was in incorrect format

Workaround:

• To add a label or field as a placeholder for the parent key on each row in the CSV file.
• Issue Search using JQL to find the issues with the label or a placeholder for the parent key.
• Bulk Edit the parent key and clear the label or placeholder field.

OR

• Use automation to achieve the same result instead of Bulk Edit.

OR

• Create a Temporary Company Managed project import Work Items via CSV import.
• Bulk Move Work Items from Temporary Company Managed Project to Team Managed Project.