The error message you're seeing ("Cannot create a partition without any constraint") is trying to tell us that in order to define a hierarchical module as a partition, you must first define a *placement* constraint for the module within the design. A placement constraint is a guide, region, or fence- in the case of a partition it should be a fence. You can use "createFence" to do this, or via the GUI by dragging the pink module guide from the left of the design into the core area and then calling "definePartition" (or Partition->Specify Partition... via the GUI).Originally posted in cdnusers.org by BobD
One other thing comes to mind based on your additional problem description (thanks for that- it's very helpful) is that you might want to try defining your AutoCTSRootPin on the output of an instance within the digital block instead of on the output of a top-level IO. If there is not an instance within the digital block that makes this approach possible, you could add a new buffer for this purpose (using Encounter ECO commands). If you do this, it's possible that the buffers would reside within the partition without any additional effort on your part.
Feel free to post back for more clarification if you'd like.