This error is being caused by HEAT looking for a subscription in SSRS that does not exist. This can happen in a variety of circumstances, including when a subscription is manually deleted in SSRS or when SSRS was reinstalled.
Steps to re-sync the HEAT and SSRS to get error to disappear:
-Go to the Report workspace in HEAT
-Go into the report with the subscription error
-Schedule a report subscription and save. This will overwrite the old subscription entry with a new entry that HEAT and SSRS are both aware of.
-Go back into the report and remove the subscription