This likely means that you are trying to deliver a service outside of the Service Start and End dates. Services can only be delivered within the date range of the service listed on the Future IEP. The calendar will gray out if the service delivery date is outside of that date range. 

If the service dates on the affirmed IEP are incorrect, you will need to amend the service via the amendment or add a new service with the correct service dates. If the service has not yet been affirmed, then you can edit it on the Future IEP.