Clarity

When saving Microsoft project with sub-tasks, user gets the message "Summary task assignments not supported", then a "failed to save" message. The project has no assigned tasks.

If all tasks are moved to the top level in Microsoft project, it will save to PPM, but that is not manageable for Project Managers.

What is causing this issue?

Unlike MS Project in CA PPM  you can only assign resources to the lowest level WBS items which are by default labeled tasks. No can do anything else. One level only

You cannot create sub tasks below task level.

If you do want to have subtasks you have to relabel the task level and create a new summary level  above that and label that as task.

However, in that case you can only assign resources to the subtasks.