The envelope number is "pending reservation"
The software automatically will put an envelope number in reserve for a period of time (often an hour) if that number was removed/edited from a different family. This can also happen when there is a slight interruption in the envelope assigning process.
When the server refreshes, as it does about every hour or so, it will search your Family Directory to ensure that the envelope number is not assigned to another family. If no other family has that number, the number will be released from the reservation system and will be able to be assigned to the desired family record.
If it has been an hour or 2 since you tried to assign the envelope number to the desired family, go ahead and navigate to the family record and try to add the envelope number. It takes 24 hours for the Pending Reservation error message to go away. If the problem persists 24 hours after you first tried to input the envelope number, please contact ParishSOFT Support.