Typically this happens at the start of a new fiscal year.  You need to run the process to Copy Prior Year Targets or Set Strategy Targets.  This can be found on the Process tab in the Asset Rebalancing Super Tab in the General Ledger.

It is also possible to have a blank report if there really are no assets to move.  This typically occurs when you run asset rebalancing again before any journal entries impacting the asset accounts are posted.