The date for this service has not changed in Companies House yet, so this service cannot be completed. Try again later.
The error means exactly that: you've filed something with CH, could be a CS01 or accounts, but they've not been accepted and processed yet, meaning CH have not yet issued the new dates.
So as a failsafe, Engager won't allow you to close out a Service until these documents are accepted and new dates provided. It's there to save you just in case the accounts get rejected and you miss this, so although a slight annoyance, it's there to protect you.