At this time, this error appears to be cosmetic, for the Software Update Cycle still runs when the Maintenance Window opens.
Research found that this error is also thrown when the Start date is chronologically after the End date as configured on the Default Software Update Plug-in (DSUP) Policy, and the Software Update Cycle will not execute if there is no Maintenance Window present, for the schedule is essentially NULL.
Found if disabling the End date is in place, or the End date is chronologically after the Start date; the process works and the schedule is received.
Note: Confirmed in Patch Management 8.x that the setting configured with the Start date is chronologically after the End date the DSUP Policy will not even save and errors with the following GUI notifications:
No Software Update Installation schedule set
The policy schedule can not be applied, if start date later than end date
Note: This functionality is not best practices, for the Default Software Update Plug-in Policy is the preferred method for executing the Software Update Cycle. This functionality is not present in Patch Management 7.1, for the client will fail to update configuration if the schedule is deleted from the Default Software Update Plug-in Policy.
Review the following relating articles to gain a better understanding of possible conflicts regarding the Software Update Cycle: